Forces and equilibrium — practice question

A rough plane is set at an angle $\alpha$ to the horizontal, with $\tan\alpha = 2.4$. A small block of mass $0.6\,\text{kg}$ is kept at rest on the plane by a horizontal force of magnitude $P\,\text{N}$. This force lies in a vertical plane passing through a line of greatest slope (see diagram). The coefficient of friction between the block and the plane is $0.4$. The block is just about to slide down the plane.