13. \(\mathrm { f } ( x ) = x ^ { 3 } - x ^ { 2 } - 7 x + c\), where \(c\) is a constant.
Given that \(\mathrm { f } ( 4 ) = 0\),
- find the value of \(c\),
- factorise \(\mathrm { f } ( x )\) as the product of a linear factor and a quadratic factor.
- Hence show that, apart from \(x = 4\), there are no real values of \(x\) for which \(\mathrm { f } ( x ) = 0\).