Just a quick question because I can't quite seem to figure this out.

They give you the equation f(x) = 1/6 [x^2 -4x +24].

They then ask you to give the inverse function f-1(x) for which the domain is positive.

It is then given that N is a negative real number not in the domain of f-1(x), asking you to find f-1(f(x)). I sub in the value for f(N) into f(x) then sub that back into f-1(f(x)) but it obviously just gives the number N :S.

Just wondering how people here would approach this question.

The answers give the answer as 4-N :S.

Thanks in advance .