So you can you the Pythagorean theorem to solve this problem.
if each side of the square is a certain length x, then the square is made up of two right triangles.
the sides adjacent to the right angle are length x while the hypoteneus is the diagonal whcih measures 36 feet.
a^2 + b^2 = c^2
x^2 + x^2 = 36^2
2x^2 = 1296
x^2 = 648
x = 25.456
area of a square = x^2 = 648 ft^2