width = w, length = l, P = 8350 (perimeter), A = total area. Units are everywhere feet.
P = 2w + 3l (That's 2w + 2l to bound the area, and another fence of length l down the middle to split the fields.)
Thus l = (P-2w)/3.
Note w>0, l>0. Thus also (P-2w)/3 >0, so w < P/2.
A = lw = w(P-2w)/3.
Translation: Maximize A(w) = w(P-2w)/3 where 0<w<P/2, and where P is constant.