Functions f and g are defined by

\(f : x \mapsto 2x + 3\) for \(x \leq 0\),

\(g : x \mapsto x^2 - 6x\) for \(x \leq 3\).

(i) Express \(f^{-1}(x)\) in terms of \(x\) and solve the equation \(f(x) = f^{-1}(x)\).

(iii) Find the set of values of \(x\) which satisfy \(gf(x) \leq 16\).