List the points in the xy-plane, if any, at which the function z = 9+ V (x+4) + (y 2) is not differentiable. points = (-4,2) (If there is more than one point, give them as a comma-separated list, e.g., (1,2), (3,4); if the function is differentiable everywhere, enter the word none.)

List the points in the xy-plane, if any, at which the function z=9+(x+4)2+(y2)2 is not differentiable. points = (If there is more than one point, give them as a comma-separated list, e.g., (1,2),(3,4); if the function is differentiable everywhere, enter the word none.)