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### Course: Algebra I (2018 edition) > Unit 5

Lesson 10: Summary: Forms of two-variable linear equations# Slope from equation

Worked examples of finding the slope of a line given its equation, using many forms of equations.

## Want to join the conversation?

- I don't quite understand where Sal got the equation in0:27. He mentioned other videos; where can they be found?(31 votes)
- It is a form of linear equation called slope-intercept form.

videos:

https://www.khanacademy.org/math/algebra/x2f8bb11595b61c86:forms-of-linear-equations/x2f8bb11595b61c86:intro-to-slope-intercept-form/v/slope-intercept-form(35 votes)

- Guys, I'm going to be honest with you.

I am completely LOST.

Does anyone have a different way to explain things?

I was following along with Sal and kinda starting to understand things until4:03.(25 votes)- Im very confused too...(9 votes)

- the video starts with a review of the y=mx+b formula which, according to Sal, was covered in previous videos;

it wasn't...

i think the order of these playlists is sometimes messed up(20 votes)- It was... See this link for the lessons on forms of linear equations. The first set of videos & exercises covers slope intercept form: https://www.khanacademy.org/math/algebra/x2f8bb11595b61c86:forms-of-linear-equations(12 votes)

- If we had a problem such as 4x-3y=9, and we needed to find the slope of that line, how would we do that, since you'd need to move the y onto the other side?(5 votes)
- Subtract 4x from both sides.

Then, divide the entire equation by -3 (the coefficient of Y).

The equation will then be in slope-intercept form: y = mx+b

The slope will be the coefficient of X.

Hope this helps.(15 votes)

- How is point-slope form helpful for me if the y = mx + b mean the same?(6 votes)
- Point-slope is mostly used when the point you're given is not the y-int.(8 votes)

- what is point slope form??(3 votes)
- Point-Slope form of an equation is a variation of the slope formula.

Slope formula: m = (y2-y1)/(x2-x1)

Point-Slope: y-y1 = m (x-x1)

Basically, the slope formula has been multiplied on both sides by (x2-x1). Then the x2 and y2 have been changed to just x and y. This form of a linear equation is useful when you are creating the equation of a line. All you need is the slope of the line (m) and one point from the line (x1, y1).

You can learn more at: https://www.khanacademy.org/math/algebra/x2f8bb11595b61c86:forms-of-linear-equations/x2f8bb11595b61c86:point-slope-form/v/idea-behind-point-slope-form(13 votes)

- The videos introducing Point-Slope and Slope Intercept form are in the Algebra playlists.

Algebra Unit 5 Lesson 4

Here's the link...

https://www.khanacademy.org/math/algebra/x2f8bb11595b61c86:forms-of-linear-equations/x2f8bb11595b61c86:point-slope-form/v/idea-behind-point-slope-form(9 votes) - what is "b"? when Sal said y=mx+b(2 votes)
- In slope-intercept form (y=mx+b), the "b" is the y-intercept at (0,b). For example: y=2x-5, the 2 is the slope of the line and the -5 if the y-intercept at (0,-5)(11 votes)

- Is this a introduction to calculus?(5 votes)
- Interesting question!

The slope of a straight line is not yet an introduction to calculus, but the slope at a single point on a curve**would**be an introduction to calculus.(6 votes)

- I'm not really understanding this. I start to think it makes sense until I have to do problems on my own and then it's like I don't even know where it start. Can someone please explain this to me step by step?(6 votes)

## Video transcript

- [Instructor] We've got the
equation Y plus two is equal to negative two, times X minus three. And, what I wanna do is figure out what is the slope of the line
that this equation describes? And there's a couple of ways
that you can approach it. What my brain wants to do is well, I know a few forms where it's easy to pick out the slope. For example, if I can manipulate that equation to be in the form Y is equal to MX plus B, well then I know that this M here, the
coefficient on the X term, well that's going to be my slope. And B is going to be my Y intercept, we cover that in many other videos. Another option is to get
into point-slope form. So the general framework
or the general template for point-slope form is,
if I have an equation of the form Y minus Y1
is equal to M times X minus X1, well then I immediately know that the line that this equation describes is going to have a slope of M once again. And here the Y intercept
doesn't jump out at you. Let me make sure you
can read this over here. The Y intercept doesn't jump out at you, but you know a point that is on this line. In particular, you know
that the point X1, Y1 is going to be on this line. X1, Y1. So let's look at our original example. So it might immediately jump out at you that this is actually
in point-slope form. You might say, well okay, I see I have a minus X1, so X1 would be three, I have my slope here and
that answers our question, our slope would be negative two. But here it says plus two,
I have to subtract a Y1. Well, you could just rewrite this, so it says, so you have
Y minus negative two is equal to negative
two times X minus three, and then you see it's exactly this point-slope form right over here. So our slope right over
there is negative two, and then if I were to ask you, well give me a point
that sits on this line, you could say, alright,
an X1 would be three, and a Y1 would be negative two. This point sits on the line,
it's not the Y intercept, but it's a point on the line and we know the slope is negative two. Now another way to approach this is to just manipulate it so that we get into slope-intercept form. So let's do that, let's manipulate it so we get into slope-intercept form. So the first thing my brain wants to do is distribute this negative two, and if I do that, I get Y plus two is equal to negative 2x, negative two times negative three, plus six. And then I can subtract
two from both sides, and then I get Y is equal
to negative 2x plus four. And so here I am in
slope Y intercept form, and once again, I could
say, alright my M here, the coefficient on the
X term, is my slope. So my slope is negative two. Let's do another example. So here, this equation
doesn't immediately go into either one of these
forms, so let's manipulate it. And if it's in either one of them, I like to get into slope Y intercept form, it's a little bit easier
for my brain to understand. So let's do that. So let us collect, well let's get the Xs, let's just isolate the Y
on the right-hand side, since the 2y is already there. So let's add three to both sides. I'm just trying to get rid
of this negative three. So if we add three to both sides, on the left-hand side we
have negative 4x plus 10 is equal to 2y, these cancel out. That was the whole point. And now to solve for Y, we just have to divide both sides by two. So if we divide everything by two, we get negative 2x plus
five is equal to Y. So this is in slope-intercept form. I just have the Y on the
right-hand side instead of the left-hand side. We have Y is equal to mx plus B, and so our M is the coefficient on the X term right over here. So our slope is once
again, is negative two, and here our Y intercept is five, in case we wanted to know it. Let's do one more
example, one more example. Alright, so once again, this
is in neither slope-intercept or point-slope form to begin with, so let's just try to get
it to slope-intercept form. And like always, pause the video, and see if you can figure it out yourself. Alright, so let's get all the Ys on the left-hand side isolated, and the Xs on the right-hand side. So let me get rid of this negative 3x. So I'm gonna add 3x to both sides, and let's get rid of
this 3y right over here. So let's subtract 3y from both sides. You couldn't do this, I'm
doing two steps at once, but once again, I'm trying to get rid of this 3x, so I'm trying to get rid of this negative 3x, so I add 3x to the left, but I have
to do it to the right if I want to maintain the equality. And if I want to get rid of this 3y, well I subtract 3y from here, but I have to do it on the left-hand side if I want to maintain the equality. So what do I get? That cancels out, 5y minus 3y is 2y, is equal to 2x plus 3x is 5x, and then these two characters cancel out. And so if I want to solve for Y, I just divide both
sides by two and I get Y is equal to five halves X, and I'm done. And you mights say wait,
if this doesn't look exactly like slope-intercept form, where is my B? Well your B, if you wanted to see it, you could just write plus zero, B is implicitly zero right over here. So your slope, your slope is going to be the coefficient on the X term, it's going to be five halves. And if you want to know your Y intercept, well it's zero, when X is zero, Y is zero.