The length of a rectangular box is 2 units less than its width.
if the area of the box is 15 units square.
find the perimeter of the box.

Length, l
width, w

l = w - 2

15 = lw
15 = (w-2) w
w^2 - 2w = 15
w^2 - 2w - 15 = 0
w^2 - 5w + 3w - 15 = 0
w (w - 5) + 3 (w - 5) = 0
(w + 3) (w - 5) = 0
w = 5             or            w = -3
possible                        not possible

width = 5 units
length = 3 units

The answer would be the second choice : )