Considering pulley 1 which is moving v1=vP+vQ2=vA (point P is not moving, therefore vP=0) ⇒3=0+vQ2 (taking downward velocities positive and upward negative) ⇒vQ=6m/s
Considering pulley 2 which is fixed. ∴vE=vD=vQ=6m/s
Now consider pulley 3 which is moving v3=vF+vG2=vB (here vF=vE=6m/s) ⇒5=−6+vG2 (taking downward velocities positive and upward negative) ⇒vG=16m/s
Since pulley 4 is also fixed, vJ=vI=vH=vG=16m/s Therefore the velocity of block C is 16m/s