The graph of $f$ is given. State the numbers at which $f$ is not differentiable. (Enter your answers as a comma-separated list.) (i) \[ x= \]

Step 1 ：To find the points where the function is not differentiable, we need to look for sharp turns, corners, or discontinuities in the graph. Let's assume there are three points A, B, and C where the function is not differentiable.

Step 2 ：We need to find the x-coordinates of points A, B, and C.

Step 3 ：points = [2, 4, 6]

Step 4 ：\(\boxed{x = 2, 4, 6}\)