Probability — practice question

A small ball $B$ is joined to one end of a light elastic string whose natural length is $0.4\,\text{m}$ and whose modulus of elasticity is $12\,\text{N}$. The opposite end of the string is fixed at point $A$. The ball is thrown vertically downwards at speed $1\,\text{m s}^{-1}$ from a point $0.4\,\text{m}$ vertically beneath $A$, and it attains its maximum speed at the position $0.7\,\text{m}$ below $A$.