How to prove that $ f (x) = (x ^ 2-9) cdot D (x) + 9 $ is not differentiable for each $ x in Bbb R $

Let $ f (x) = (x ^ 2-9) cdot D (x) + $ 9

Question: How to prove this $ f $ is not differentiable for all $ x in Bbb R $

Can anyone help me?