A rectangle’s perimeter is determined by the formula P = 2(l + w). The length of this rectangle is 3 + 2w. Substituting the known values into the formula results in 36 = 2(w + 3 + 2w).
36 = 2(3w + 3)
18 = 3w + 3
15 = 3w
w = 5
As the length is 3 + 2w, then l = 3 + 2(5), or l = 13.