consider the figure shown and observe carefully the forces shown....

for the body of mass M ....

Mg - T = M.2a _________(1)

 

and as  the pulley B is massless .... So force acting on it must be zero....

Such that tension in the string connecting B and 2M is T+T = 2T

 

So 2T = 2M.a _______(2)

 

Comparing the two equations ....

Mg - T = 2T

ot T = Mg/3

 

So (1) => Mg - Mg/3 = 2Ma

or a = g/3

 

Now accl^n of M = 2a = 2g/3

 

And force due to clamp on pulley = force on clamp due to pulley... = resultant of the two tensions in the string going over the pulley A

(the string connected to the clamp has no effect due to the pulley on the clamp...but itself pulls the clamp..... and so is not to be counted)

 

So reqd. force due to pulley on clamp = ( T² + T² )

                                                       =  2T²

                                                     = T2

                                                       = Mg/3 . 2 ( 45⁰ with horizontal as the two forces are equal in magnitude and are mutually perpendicular )