Two blocks of mass 3kg and 6kg respectively are placed on a smooth horizontal surface and they are connected by a light spring of force constant $k=200 N{m}^{-1}$. Initially, the spring is unstretched and velocities of  $1 m{s}^{-1}$ and $2 m{s}^{-1}$ are imparted in opposite directions to the respective blocks as shown in figure. The maximum extension of the spring will be