A man is pulling the rope from both sides with constant speed u.
According to figure,
Applying of virtual work method
∑→T⋅→V=0
Let the velocity of mass M be v in upward direction.
Now,
−Tu−uT2+2Tv=0
−2Tu−Tu2=−2Tv
−3Tu=−4Tv
v=3u4
Hence, the speed of the block will be 3u4.