Let l = the length, w = the width.

If the length is 4 more than the width, then l = w+4 | w we do not know.

The parameter is given to be 40 so, you have 2 lengths and 2 widths.

w + (w+4) + w + (w+4) = 40

4w+8 = 40

4w = 32

w = 8

So the width (w) is 8, since the length (l) is 4 more the width (w) it must be 12. 8 + 12 + 8 12 = 40