Given function is f(x)={\begin{matrix}max(|x|,{x}^{2}), & |x|\leq 2 \\ 8-2|x|, & 2<|x|\leq 4\end{matrix}
The graph of the function, is

We know that, a function is not differentiable at a point where its graph has a sharp corner.
And, from the graph we can easily conclude that f(x) is non-derivable at x=−2,−1,0,1,2
Hence, the set S=−2,−1,0,1,2.