(i)[3]
Find how long after projection $P$ first strikes the ground, and determine the distance $OA$.
(ii)[5]
When $P$ bounces at $A$, the horizontal component of $P$'s velocity is unchanged. Immediately after the bounce, the vertical component of velocity is $8\,\text{m s}^{-1}$. $P$ hits the ground a second time at $B$, where it comes to rest. Calculate the first and last times after projection at which the speed of $P$ is $18\,\text{m s}^{-1}$.