Probability — practice question

A particle $P$ with mass $0.3\,\text{kg}$ is fastened to one end of a light elastic string whose natural length is $0.6\,\text{m}$ and modulus of elasticity is $24\,\text{N}$. The other end of the string is attached to a fixed point $O$. Particle $P$ is released from rest at the point $0.4\,\text{m}$ vertically below $O$.