All you need to do is find the square root of 81, because the formula for the area of square is [tex] s^{2} [/tex].
The square root of 81 is 9 ( 9 x 9 = 81)
To double check, plug 9 into the formula.
area = [tex]s^{2} [/tex]
area = [tex] 9^{2} [/tex]
area = 81
So, therefore, each side is 9ft.