The function f is defined by

\(f : x \mapsto 3x - 2\) for \(x \in \mathbb{R}\).

The function g is defined by

\(g : x \mapsto 6x - x^2\) for \(x \in \mathbb{R}\).

Express \(gf(x)\) in terms of \(x\), and hence show that the maximum value of \(gf(x)\) is 9.