Consider the function $f(x) = 2 -6 x^2$ on the interval $[ -5 , 7 ]$.
(A) Find the average or mean slope of the function on this interval, i.e.

(B) By the Mean Value Theorem, we know there exists a $c$ in the open interval $(-5, 7)$ such that $f'( c)$ is equal to this mean slope. For this problem, there is only one $c$ that works. Find it.

$c$ =