Let the length = x
2 lengths are 2x.
Then you have 400 - 2x for both widths, so the width is 200 - x.
The are if the rectangle is
y = x(200 - x)
y = 200x - x^2
y = -x^2 + 200x
Take the first derivative ans set equal to zero to find a maximum value.
y' = -2x + 200
-2x + 200 = 0
-2x = -200
x = 100
Since the side of the rectangle is 100, all sides measure 100 ft, and you have a square.
The maximum area is 100 ft * 100 ft = 10,000 ft.