The perimeter of the rectangle is 2(L+W) where L and W are length and width. So L+W=250'. W=250-L.
Area=L(250-L). This area is maximum when L=125, area=125^2=15625 sq ft. (rectangle is a square).
To prove this let L=125+a and W=125-a. Perimeter=2(125+a+125-a)=500.
Area=125^2-a^2, which is always <125^2 except when a=0. So the maximum area is 125^2.