The answer is show that the slope of line SQ is 1 and the slope of line PR is -1.
Suppose a square has the following coordinates (0,0), (a, 0), (0, a), and (a,a).
The diagonals are distinct by (0,0) to (a,a) and (a,0) to (0,a)
The slope from (0,0) to (a,a) = (a - 0)/(a - 0) = a/a = 1 The slope from (a,0) to (0,a) = (0 - a)/(a - 0) = -a/a = -1
Two lines are thought to be perpendicular when their slopes are negative reverses of each other. In math, m2 = -1/m1
1 = -1/-1, thus these two lines, the diagonals of the square, are perpendicular.