When simplifying this I would notice that one side is subtracting 2 (a rational number) and that the other also has a rational number. To simplify this, you have to get rid of the -2 by canceling it out. You can do this by adding 2 on both sides (you have to add 2 to both sides to make sure both sides of the equal sign are equal).
So instead of subtracting 2 on one side and subtracting 2 on the other, you are supposed to add 2 to both sides. They solved the second step correctly
