3 The polynomial \(\mathrm { f } ( x )\) is defined by
$$f ( x ) = x ^ { 3 } + a x ^ { 2 } - a x + 14$$
where \(a\) is a constant. It is given that ( \(x + 2\) ) is a factor of \(\mathrm { f } ( x )\).
- Find the value of \(a\).
- Show that, when \(a\) has this value, the equation \(\mathrm { f } ( x ) = 0\) has only one real root.