What Conditions Make a∘b Equal to b∘a for Given Functions?

[cahiersujet]

Homework Statement

Let a=fx+g and b=hx+i. For which real numbers f,g,h,i a∘ b = b∘ a

Homework Equations

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

 

[The Bob]

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