Consider the situation shown in figure. All the surfaces are smooth. The tension in the string connected to 2 m is
mg3
4mg3
2mg3
mg
aA = aB+02
2aA = aB
maB = m(2aA)------(i)
2T = 2m.aA---------(ii)
From 2(i) + (ii), we get
2mg = 6maA
T' = 2maA = 2mg3