# 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?