3.120: Piecewise acceleration; velocity and displacement

A particle $P$ moves in a straight line such that, $t$ seconds after passing a fixed point $O$, its acceleration $a\,\mathrm{ms}^{-2}$ is
$$a = 6t\quad(0 \leqslant t \leqslant 3),\qquad a = 18\mathrm{e}^{3 - t}\quad(t \geqslant 3).$$
When $t = 1$, the velocity of $P$ is $2\,\mathrm{ms}^{-1}$ and its displacement from $O$ is $-4\,\mathrm{m}$.