pls help me. I have out of idea how to get started

Quote:

Consider the surface http://webwork.asu.edu/webwork2_file...dfa9d84bd1.png

Describe the set of points on the surface for which it is not possible to define the surface as the graph of a differentiable function http://webwork.asu.edu/webwork2_file...f642098b81.png.

Your answer should be in the form http://webwork.asu.edu/webwork2_file...b9cd377431.png

__________________http://webwork.asu.edu/webwork2_file...c290a922d1.png