11 The function f : \(x \mapsto 6 - 4 \cos \left( \frac { 1 } { 2 } x \right)\) is defined for \(0 \leqslant x \leqslant 2 \pi\).
- Find the exact value of \(x\) for which \(\mathrm { f } ( x ) = 4\).
- State the range of f.
- Sketch the graph of \(y = \mathrm { f } ( x )\).
- Find an expression for \(\mathrm { f } ^ { - 1 } ( x )\).