Composition of functions

1. The problem statement, all variables and given/known data
Let a=fx+g and b=hx+i. For which real numbers f,g,h,i a∘ b = b∘ a

2. Relevant equations

3. The attempt at a solution
a∘ b(x)=(a(b(x))= h(fx+g)+i= fhx+gh+i
b∘ a(x)=(b(a(x))= f(hx+i)+g= fhx+fi+g
I'm kind of stuck here as I don't think I know what the question is really asking for so I'm unable to proceed :S
Equate the two, rearrange and find solutions, even if algebraic, for the unknowns you want. Just be careful because the question seems to be asking for the whole of the real numbers.

The Bob

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