I just need to know how what I have to do in each situation1. I'm given a piece-wise function graph and i'm to determine the value(s) of x where the function is not differentiable. How would i do this?

2. y $\displaystyle =(x^2-4)/(x^2-2x-8)$. Determine the value(s) of x where the function is not differentiable.

3. At what points does the curve y $\displaystyle =x^3+3x^2-25x+1$ have horizontal tangents?