3 The cubic polynomial \(2 x ^ { 3 } + a x ^ { 2 } - 13 x - 6\) is denoted by \(\mathrm { f } ( x )\). It is given that ( \(x - 3\) ) is a factor of \(\mathrm { f } ( x )\).
Find the value of \(a\).
When \(a\) has this value, solve the equation \(\mathrm { f } ( x ) = 0\).