Consider the function $f(x) = 2 x^3 - 4 x$ on the interval $[ -2 , 2 ]$.
(A) Find the average or mean slope of the function on this interval.

Average Slope =
(B) By the Mean Value Theorem, we know there exists at least one $c$ in the open interval $( -2 , 2 )$ such that $f'( c)$ is equal to this mean slope. Find all values of $c$ that work and list them (separated by commas) in the box below.

List of values: