A function $f(x)$ and interval $[a, b]$ are given. Check if the Mean Value Theorem can be applied to $f$ on $[a, b]$. If so, find all values $c$ in $[a, b]$ guaranteed by the Mean Value Theorem
Note, if the Mean Value Theorem does not apply, enter DNE for the $c$ value.

$$f(x) = 14 x^2 - 14 x + 6 \qquad \textrm{on} \; [7, 15]$$ $c=$ (Separate multiple answers by commas.)