The figure shows the lengths, in centimeters (cm), of the edges of a right rectangular prism. The length of a is 3 cm and the length of b is 4 cm. The volume of the prism is 72 cubic centimeters.
The volume $V$ of a right rectangular prism is $\ell wh$, where $\ell$ is the length of the prism, $w$ is the width of the prism, and $h$ is the height of the prism.

What is the length, in centimeters, of c?