lets l to be lenght, w width, p perimeter and a area:
a=lw
6400 ft²=lw
2(l+w)=p
2(l+w)=400 ft
l+w= 200 ft
l=200ft-w
substitute it instead of l to lw=6400ft²
(200-w)w=6400
200w-w²=6400
w²-200w+6400=0
(w-160)(w-40)=0
w = 160ft
w = 40ft
l=200-w
l=200-160 = 40ft
l= 200-40 =160ft
