5 The function f is defined by \(\mathrm { f } : x \mapsto a x + b\), for \(x \in \mathbb { R }\), where \(a\) and \(b\) are constants. It is given that \(f ( 2 ) = 1\) and \(f ( 5 ) = 7\).
- Find the values of \(a\) and \(b\).
- Solve the equation \(\operatorname { ff } ( x ) = 0\).