Perimeter is the sum of all sides of a shape. For a rectangle, it is the sum of the two lengths and two width measurements. We set up the equations as follows:
l = 2w - 3
P = 8l - 12
P = 2w + 2l
8l -12 = l - 3 + 2l
5l = 9
l = 9/5
9/5 = 2w - 3
w = 12/5