Forces and equilibrium — practice question

A particle $P$ with mass $m\text{ kg}$ is in equilibrium on a rough plane tilted at an angle $\theta^{\circ}$ to the horizontal. The equilibrium of $P$ is sustained by a force of magnitude $8mg\text{ N}$, which makes an angle $\theta^{\circ}$ with the line of greatest slope (see diagram). The coefficient of friction between $P$ and the plane is $0.5$, and $P$ is just about to slip down the plane.