Let's call the rectangle's length x and its width, x - 6.
Since a rectangle has 4 sides and the perimeter of the rectangle
is just the distance around the outside of the figure,
we can create an equation.
This equation will be x + x + x - 6 + x - 6 = 72.
Simplifying on the left gives us 4x - 12 = 72.
Add 12 to both sides to get 4x = 84.
Now divide both sides by 4 to get x = 21.
Since x represents our length, we know that the length is 21 inches.