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### Course: Algebra (all content)>Unit 3

Lesson 1: Two-variable linear equations intro

# Solutions to 2-variable equations: substitution (old)

An old video of Sal checking whether (3,-4) is a solution of 5x+2y=7 by substituting x=3 and y=-4. Created by Sal Khan and Monterey Institute for Technology and Education.

## Want to join the conversation?

• why do you add 8?
• you do not add 8 you actually subtract 8.
because when you plus a negative number, it is just like subtraction
(1 vote)
• Which ordered pair is on the graph of the equation 2x+5y=4??
• There are an unlimited number of points on a line. To ask which ordered pair is on the line itself, you would have to give options.
• what do ordered pairs have to do with the math in the real world i just don't get it?
• How do you do linear equations with 0,0 as your ordered pair
• You just simply plug in (0,0) into the equation and get an answer
(1 vote)
• Simply put a direct relationship between two variables is one in which if you divide one by the other you always get the same exact number for all non-zero values of the two variables.

An inverse relationship is one in which if you multiply the two variables you always get the same exact number for all non-zero values of the two variables.
(1 vote)
• If you had and equation that wasn't in slope-intercept form, then would you have to put the equation in slope intercept form?
• Gropkins,
You do not have to put an equation in slope intercept form to graph the equation (unless that is the assignment).
As long as you are sure it is a linear equation, you graph the equation by finding any two points on the line and then plotting the two points and drawing a line through the points. You can find a point by choosing any value for x and finding the y value without putting the equations in slope y-intercept form. Choose a different value for x and find another point the same way.

But if the equation is put in slope y-intercept form, you can graph it by plotting the y-intercept and then using the slope to find a second point.

I hope that helps make it click for you.
• Which ordered pair is not included in the graph of p=3m+6? (1,9) , (3,12) , (10,36) , (6,18)
(1 vote)
• I won't give the answer, but I'll tell you how to do it yourself. In this equation, p = y and m = x. So, with each of your ordered pairs, plug them into the equation and calculate it. If both sides are equal, the pair is included in the graph. If they aren't equal, then the pair is not in the graph.

For example, the first coordinate (1, 9).
9 = 3(1) + 6.
9 = 3 + 6
9 = 9
So (1, 9) is included in the graph.
• When your typing out the ordered pair do you put the largest number first?
• No. Ordered pairs are always (x, y). x always goes first and y always goes second. If you didn't put them in that order, then you wouldn't know which was which.