Use the definition of the derivative:
As long as h is not 0, so .
for all non-zero h while h goes to 0 so the derivative at x= 0 is .
Notice that for x not 0, the derivative is given by the product rule:
Now, that has no limit as x goes to 0 so while f is differentiable for all x, its derivative is not continuous at x= 0. That means that f' is not differentiable at x= 0 and so f is not twice differentiable at x= 0.