Answer
$f = -x^{2} $
$c = 6$
Work Step by Step
We have that $f'(c) = \lim\limits_{x \to 6}$ $\frac{-x^{2} + 36 }{x - 6}$.
Taken the original formula $f'(c) = \lim\limits_{x \to c}$ $\frac{f(x) - f(c)}{x - c}$, we can conclude that
$f = -x^2$ and $c = 6$
