Newton's laws of motion — practice question

$A$, $B$ and $C$ are three points along the line of greatest slope on a plane inclined at $\theta^\circ$ to the horizontal, with $A$ above $B$ and $B$ above $C$. The section between $A$ and $B$ is smooth, while the section between $B$ and $C$ is rough. A particle $P$ is released from rest at $A$ and slides down the line $ABC$. $0.8\,\text{s}$ after leaving $A$, the particle passes through $B$ with speed $4\,\text{m s}^{-1}$.