Question: How to create a function of a function?

Hi everyone,

 

I would like to do:

f:=unapply(x**2,x);
j:=unapply(2*f(x),f(x));
 
Basically it is to define j(f(x))
But this is doesn't work. What is the right way to do it?\
 
My final goal is to calculate int(j(f(x)), f(x)=-infinity..infinity).
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