Lets assume that there are 2 lines from the point Q that are perpendicular to the diagonals of the square ( QM and QN ).
QM = 2 ft and QN = 3 ft.
Then : QM + QN = d/2 ( d is the diagonal )
2 + 3 = d/2
d = 5 * 2
d = 10 ft
Answer: The diagonal of the square is 10 ft long.