Answer
$f = 2\sqrt x$
$c = 9$
Work Step by Step
We have that $f'(c) = \lim\limits_{x \to 9}$ $\frac{2\sqrt x - 6}{x - 9}$.
Taken the original formula $f'(c) = \lim\limits_{x \to c}$ $\frac{f(x) - f(c)}{x - c}$, we can conclude that
$f = 2\sqrt x$ and $c = 9$
