In order to determine the correct expression for the length of the side, consider that the distance in between two points (x1,y1) and (x2,y2) is given by the following formula:
d = √((x2 - x1)² + (y2 - y1)²)
if (x1,y1) = (5,5) and (x2,y2) = (7,-3) you have for d:
d = √((7 - 5)²+(5 - (-3))²)