My question is about QFT

I'm reading mandle and shaw

in chapter 2 as you know there is a question (2.4) about the commutations between the field and momentum.

[ [P]^{}[j], [tex]\phi[/tex] ]

as momentum is in integral form I don't know how to prove them!

I tried to open the terms but I don't know how to put the field in an intergal!