3. A particle \(P\) of mass 0.4 kg is moving under the action of a constant force \(\mathbf { F }\) newtons. Initially the velocity of \(P\) is \(( 6 \mathbf { i } - 27 \mathbf { j } ) \mathrm { m } \mathrm { s } ^ { - 1 }\) and 4 s later the velocity of \(P\) is \(( - 14 \mathbf { i } + 21 \mathbf { j } ) \mathrm { m } \mathrm { s } ^ { - 1 }\).

1. Find, in terms of \(\mathbf { i }\) and \(\mathbf { j }\), the acceleration of \(P\).
2. Calculate the magnitude of \(\mathbf { F }\).