Probability — practice question

A particle $P$ with mass $M\,\text{kg}$ is fastened to one end of a light elastic string whose natural length is $0.8\,\text{m}$ and whose modulus of elasticity is $12.5\,\text{N}$. The other end of the string is fixed at point $A$. The particle is released from rest at $A$ and moves vertically downward until it is momentarily at rest at point $B$. The largest speed of $P$ during the descent is $4.4\,\text{m s}^{-1}$, and this occurs when the extension of the string is $e\,\text{m}$.