Probability — practice question

A block $B$ with mass $3\,\text{kg}$ is fastened to one end of a light elastic string whose modulus of elasticity is $70\,\text{N}$ and whose natural length is $1.4\,\text{m}$. The opposite end of the string is connected to a particle $P$ of mass $0.3\,\text{kg}$. $B$ is stationary $0.9\,\text{m}$ from the edge of a horizontal table, and the string passes over a small frictionless pulley at the table edge. $P$ is released from rest beside the pulley and moves vertically downwards. At the first moment when $P$ is $0.8\,\text{m}$ below the pulley and still descending, $B$ is in limiting equilibrium with the part of the string attached to $B$ horizontal (see diagram).