Use the Five-Step Strategy for Solving Word Problems to Find $= 3$

Use the five-step strategy for solving word problems to find the number or numbers described in the following
exercise.
-A bookcase is to be constructed as shown in the figure below. The height of the bookcase is 3 feet longer than the length of a shelf. If 24 feet of lumber is available for the entire unit (including the shelves, but NOT the back of the bookcase) , find the length and height of the unit.

A) length $= 3$ feet; height $= 6$ feet
B) length $= 10.5$ feet; height $= 13.5$ feet
C) length $= 4.5$ feet; height $= 7.5$ feet
D) length $= 3$ feet; height $= 9$ feet