Probability — practice question

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