Answer: -2, 2
Explanation:
The derivative of a function, f(x), at a point where f(x) has a sharp turn (or cusp) does not exist.
The derivative of a function, f(x), at a point where f(x) has a vertical tangent line does not exist.
Using this information, we can see that f(x) is not differentiable when x=3 since there is a vertical tangent line. We can also see that f(x) is not differentiable when x=-1 since there is a cusp.