In the system shown in fig., all pulleys are mass less and the string is inextensible and light.

In the system shown in fig., all pulleys are mass less and the string is inextensible and light. (a) After the system is released, find the acceleration of mass `m_(1)` (b) If `m_(1) = 1 kg, m_(2) = 2 kg \"and\" m_(3) = 3 `kg then what must be value of mass `m_(4)` so that it accelerates downwards?