Please explain the problem in image fully stepwise ......#....clearly........

in the question , the block below is experiencing two forces , one due to the friction and other due to the tension in that string . also the frictional froce acts in the direction of movement of both of them on block and opposite to that direction on the man . the tension 

                                   T=100 N

                                 let n m/s^-2 be the common accelaration of the man and the block below man .

                                AND LET f BE THE FRICTION (WHICH WOULD BE SAME ON BOTH OF THE BLOCK AND MAN .

                                now by f(net external) = ma 

 

taking the block, we see , 

                                T-f = ma = 50n

and in the block , we see ,

                                T+f=ma=100n

 

adding both the equations above , we get ,

                                 2T=150n therefore, a=4/3 m/s^-2         (since T=100N)                     

 

substituting this value of n in any of the equations , you can easily get , 

                                 f = 100/3N 

 

hence , the requiered value of X in the question is 3