That is a pretty cryptic question you have posed there. I'm guessing that you want to find a critical point of the function on the closed interval [16, 25] using the Mean Value Theorem.
The Mean Value Theorem says:
Let [a, b] be a continuous function on the closed interval [a, b] and differentiable on the open interval (a, b), where . Then there exists some in (a, b) such that: