A function $f(x)$ and interval $[a, b]$ are given. Check if Rolle's Theorem can be applied to $f$ on $[a, b]$. If so, find all values $c$ in $[a, b]$ such that $f'(c) = 0$.
Note, if Rolle's Theorem does not apply, enter DNE for the $c$ value.

$c=$ (Separate multiple answers by commas.)