The general formula for the distance between two points is
Anyway, if A and B have the same x or y coordinates, this formula can be simplified. For example, in this case the two points have the same x coordinate of -8, so the following part of the formula simplifies:
So, we're left with
but the square root of a square is the absolute value of the object being squared:
which is this case means
which is the correct length of the side.