3 The polynomial \(4 x ^ { 3 } - 7 x + a\), where \(a\) is a constant, is denoted by \(\mathrm { p } ( x )\). It is given that ( \(2 x - 3\) ) is a factor of \(\mathrm { p } ( x )\).
Show that \(a = - 3\).
Hence, or otherwise, solve the equation \(\mathrm { p } ( x ) = 0\).