f(x) = 0 x=0
f''(0) = limit( (f'(x)-f'(0))/x,x, goes to 0), but we dont know f'(0)?
First thing first, do you know to find the derivative? To be differential at a point, it must be continuous, so we need to make sure the limit as f approaches zero is equal to zero. We also need to find the limit as f' approaches zero, and if that equals zero, then we are good to go.