Probability — practice question

A particle $P$ with mass $0.35\text{ kg}$ is fastened to the midpoint of a light elastic string whose natural length is $4\text{ m}$. The two ends of the string are fixed at points $A$ and $B$, which are $4.8\text{ m}$ apart and lie at the same horizontal height. In equilibrium, $P$ hangs at a point $0.7\text{ m}$ vertically beneath the midpoint $M$ of $AB$ (see diagram).