Because we are given the area, work backwards by taking the square root for each sides of formula.
A (area) = 2 * (L/Length) / or (L/Length)^2
A = L2
99 ft^2 = L^2
√99 ft^2 = √L^2
9.9498743710662 ft = L
So, Length (L) = 9.9498743710662 ft.
(this length value applies for 4 (4x) since it's a 4-equal side polygon shape)