let charge in the line of 24 volt is Q in the direction of battery and at point b, let the charges in the two lines are q and Q-q (q in the middle line)

now accoring to the Kirchhoff law

24-12 = Q/1 + q/2

and 24-6 = Q/1 + (Q-q)/4 

solve these 2 for q and Q 

then by point potential rules get the answer.