here's a short soln. i hope this helps

equation for small block

(/2)mg = ma_s    (a_s = acceleration of small block w.r.t ground)

=> a_s = (/2)g   (towards left)

similarly equation for larger block

(M+m)g - (/2)mg  = Ma_L   (where a_L = acceleration of larger block w.r.t ground )

=> a_L = [(/2)(m/M)]g + g   (towards left)

 now a_sL = a_s - aL =  (/2)g - ( [(/2)(m/M)]g + g )    (a_sL = acceleration of small block w.r.t bigger block)

 => a_sL = [{g(M+m)}/2M] (towards right)

now

l = 1/2 * a_sL * t^2

l = 1/2 * [{g(M+m)}/2M] * t^2

t^2 = 4Ml/[g(M+m)]

t = sqroot(4Ml/[g(M+m)])

hope you understod

cheers