Forces and equilibrium — practice question

A ball $B$ of mass $4\,\text{kg}$ is fastened to one end of a light inextensible string. At the opposite end of the string is a particle $P$ of mass $3\,\text{kg}$. The string runs over a fixed smooth pulley. The system is in equilibrium, with the string taut and both straight sections vertical. $B$ rests on a rough plane that is inclined to the horizontal at an angle of $\alpha$, where $\cos\alpha = 0.8$ (see diagram).