Momentum — practice question

Particles $P$ and $Q$, with masses $m\text{ kg}$ and $2\text{ kg}$ respectively, are free to move on a smooth horizontal plane. They travel directly towards one another at speeds $u\text{ m s}^{-1}$ and $3u\text{ m s}^{-1}$ respectively. A collision takes place, and the direction of motion of each particle is reversed by the impact. Immediately after the collision, the speed of $P$ is $\frac{u}{3}\text{ m s}^{-1}$.