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# One-step subtraction equations

Video transcript

Solve for a and
check your solution. And we have a plus
5 is equal to 54. Now, all this is
saying is that we have some numbers,
some variable a. And if I add 5 to
it, I will get 54. And you might be able
to do this in your head. But we're going to do it a
little bit more systematically. Because that'll be
helpful for you when we do more complicated problems. So in general, whenever you
have an equation like this, we want to have the variable. We want this a all by itself
on one side of the equation. We want to isolate it. It's already on
the left-hand side, so let's try to get
rid of everything else on the left-hand side. Well, the only other thing
on the left-hand side is this positive 5. Well, the best way to get rid
of a plus 5, or a positive 5, is to subtract 5. So let's subtract 5. But remember, this says
a plus 5 is equal to 54. If we want the
equality to still hold, anything we do to the left-hand
side of this equation, we have to do to the right
side of the equation. So we also have to
subtract 54 from the right. So we have a plus 5 minus 5. Well, that's just
going to be a plus 0. Because if you add 5 and you
subtract 5, they cancel out. So a plus 0 is just a. And then 54 minus 5, that is 49. And we're done. We have solved for a. A is equal to 49. And now we can check it. And we can check it by
just substituting 49 back for a in our original equation. So instead of writing a
plus 5 is equal to 54, let's see if 49 plus
5 is equal to 54. So we're just
substituting it back in. 49, 49 plus-- let me do that
in that same shade of green. 49 plus 5 is equal to 54. We're trying to check this. 49 plus 5 is 54. And that, indeed,
is equal to 54. So it all checks out.