Counterterms in self-energy diagram

[PineApple2]

I guess my question is pretty basic, and following a procedure in the textbook by Lahiri and Pal. You can see the relevant pages at
https://books.google.com/books?id=_UmPP8Yr5mYC&pg=PA245&source=gbs_toc_r&cad=4#v=onepage&q&f=false
On eqs. (12.84)-(12.86). I don't see how to get from (12.85) to (12.86). How do you do this differentiation?

Thanks.

 

[Avodyne]

Think of $p^2$ as $\rlap{\,/}p^2$. Then differentiate with respect to $\rlap{\,/}p$.

 

[PineApple2]

Think of $p^2$ as $\rlap{\,/}p^2$. Then differentiate with respect to $\rlap{\,/}p$.

Sure. I was not concentrated last night. Thanks!