A helicopter takes off along the vertical with an acceleration a =...

A helicopter takes off along the vertical with an acceleration $a = 3m{s}^{- 2}$ and zero initial velocity. In a certain time ${t}_{1}$, the pilot switches off the engine. At the point of take-off, the sound dies away in time ${t}_{2} = 30s$. Determine the velocity $v$ of the helicopter (in $m{s}^{- 1}$) at the moment when its engine is switched off, assuming that the velocity $c$ of sound is $320m{s}^{- 1}$.