There is a massless pulley which is going upward with an acceleration a=5tm/s2. A string is passing over the pulley as shown in the figure. At one end of the string a plank of mass 2m kg is attached and at the other end a small ball of mass m kg is attached. It is given that after 1 sec it just passes the plank then if length of plank is x + 0.8 . Find x.