If the den is in rectangular shape, the perimeter is twice the sum of the length and the width.
P = 2L + 2W
Substituting,
326 = 2L + 2W ; 163 = L + W ; W = 163 - L
Then, the area is equal to,
A = L x W = L x (163 - L)
A = 163L - L²
Getting the derivative of the area and equating the derivative to zero,
dA = 163 - 2L = 0 ; L = 81.5 ft
Thus, the dimensions of the den is 81.5 ft x 81.5 ft
The maximum area is equal to 6642.25 ft².