So in order to do this problem, we want to get y by itself. So right now, the y has x being added to it. In order to "undo" the addition, we need to subtract something from both sides that leaves us with just y on the left.

So we do:

x+y=15

-x+x+y=-x+15

y=-x+15 OR y=15-x

The answers are the same, just two ways to write them.