This is a right angle triangle and you use the Pythagorean theorem to solve for the hypotenuse (the diagonal)
A^2 + B^2 = C^2
8^2 + 8^2 = C^2
64 + 64 = C^2
128 = C^2
C = square_root (128)
C = 11 (to the nearest whole number)
The diagonal is 11 feet (to the nearest whole number)