this is of page 39 cm module cn it be done

In this problem, whenever, there is any question of change in momentum due to any hitting force, always use, IMPULSE-MOMENTUM THEOREM.
In this problem, the collision is about the inelastic collision, in which linear momentum is conserved.
You need to consider, blocks and strings as a system before collision and combined system after collision.
mu = (m+m+m)V_f