Solve the following math problem step by step. The last line of your response should be of the form Answer: \boxed{$Answer} where $Answer is the answer to the problem.

<image>In $\triangle A B C, C D$ is an altitude and $m \angle A C B=90^{\circ}$ If $A D=12$ and $B D=3,$ find $A C$ to the nearest tenth. 