Answer:
well, we only no that
2l + 2w is 80
(lengths and widths on all 4 sides
and that w * l is A
so the function should be w times what's left for l, but also expressed by something with w
2l + 2w = 80 | -2w
2l = 80 -2w | devide by 2
l = 80 - w
now we can substitute l in
A = w * l
so that we only need w's
A(w) = w * (80-w)
for any w it will give us the area, makes only sense for 0<w<80