I suppose you're given [tex]f(x) = x^2 + 2[/tex], which would mean [tex]f^{-1}(x) = \sqrt{x-2}[/tex]. Then
[tex]f^{-1}(f(x)) = f^{-1}(x^2 + 2)[/tex]
and
[tex]f^{-1}(x) = \sqrt{x - 2} \implies f^{-1}(x^2+2) = \sqrt{(x^2+2) - 2}[/tex].
You should have [tex]x^2+2[/tex] where you wrote [tex]x+2[/tex]. This then simplifies to [tex]\sqrt{x^2} = x[/tex] (as long as [tex]x\ge0[/tex]).
