Probability — practice question

A particle $P$ with mass $0.28\ \text{kg}$ is fixed to the midpoint of a light elastic string whose natural length is $4\ \text{m}$. The string ends are fastened to fixed points $A$ and $B$, which lie at the same horizontal level and are $4.8\ \text{m}$ apart. $P$ is released from rest at the midpoint of $AB$. During the later motion, the acceleration of $P$ is zero when $P$ is $0.7\ \text{m}$ below $AB$.