Forces and equilibrium — practice question

In Fig. 6.1, particles $A$ and $B$, with masses $4\,\text{kg}$ and $3\,\text{kg}$ respectively, are fixed to the ends of a light inextensible string that runs over a small smooth pulley. The pulley is attached at the top of a plane inclined at $30^\circ$ to the horizontal. $A$ hangs freely beneath the pulley, while $B$ rests on the inclined plane. The string is taut, and the part of the string from $B$ to the pulley is parallel to a line of greatest slope of the plane.