Solve the Problem $= 8.0 \mathrm { ft }$ ; Height $= 9.0 \mathrm { ft }$

Solve the problem.
-A bookcase is to be constructed as shown in the figure below. The height of the bookcase is 4 feet longer than the length of a shelf. If 20 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 $= 8.0 \mathrm { ft }$ ; height $= 9.0 \mathrm { ft }$
B) length $= 2 \mathrm { ft }$ ; height $= 8 \mathrm { ft }$
C) length $= 2 \mathrm { ft }$ ; height $= 6 \mathrm { ft }$
D) length $= 3 \mathrm { ft }$ ; height $= 7 \mathrm { ft }$