Answer:
 8.5 ft = length of diagonal
Explanation:
If A = 36 ft^2, then s (length of one side) = 6 ft.
The diagonal and any one side create a 45 degree angle. 
sin 45 degrees = (length of side opposite the 45 degree angle)/hypotenuse
 6 ft 6√2 ft 
Then (length of hypotenuse) = ------------------ = ---------------
 sin 45 deg 1
This works out to 8.5 ft = length of diagonal