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>If $\overline{MS}$ is a median of $ \triangle MNQ$, $QS = 3a - 14$, $SN = 2a + 1$, and $m∠MSQ = 7a + 1$, find the value of $a$.