To determine the x-values where the function is not differentiable, we need to look for points where the graph has any of the following:

1. Sharp corners: Where the graph changes direction abruptly, and the slope of the curve changes abruptly.
2. Vertical tangents: Where the slope of the tangent line is undefined (vertical line).
3. Discontinuities: Where the graph is not continuous, such as jumps or breaks.

In the graph you provided, you should check for these key features.

Could you describe or point out any specific features that stand out, or would you like me to guide you through it based on the given graph?