GATE CSE 2021 Set 2 Q35: Upper bound on f(3) via the mean value theorem

Suppose that $f: \mathbb{R} \rightarrow \mathbb{R}$ is a continuous function on the interval $[-3, 3]$ and a differentiable function in the interval $(-3, 3)$ such that for every $x$ in the interval, $f'(x) \leq 2$. If $f(-3) = 7$, then $f(3)$ is at most ________.