Kinematics of motion in a straight line — practice question

Particle $P$ begins from rest at point $O$ on a horizontal straight line. $P$ travels along the line with constant acceleration and arrives at point $A$ on the line with a speed of $30\,\text{m s}^{-1}$. At the moment that $P$ departs from $O$, particle $Q$ is projected vertically upwards from point $A$ with a speed of $20\,\text{m s}^{-1}$. Later, $P$ and $Q$ collide at $A$. Find