sorry i was watching an other page last time
the solution will be like this
the different lenghts l , d and y where l is lenght AM y is the distance of perpendicular drawn from mass M to the upper floor and d is distance of that point where perpendicular will meet the floor to the pulley

apply pythagorus theorm here

u will get

l2=d2+Y2 ............1(square of l equals to square of d+square of y)

differenciate it w.r.t time u will get

2l*dl/dt=2d*dd/dt

l*dl/dt=d*dd/dt............2

now as the string is pulled by velocity u so the inclined string will also move with u i.e dl/dt=u.........3 put this in 2

l*u=d*V here we have assume dd/dt=V which we require to calculate

V=u*l/d=u/cosX (in quection x is given threta)