let potential at middle junction be x. since the current coming out of two arms from point b is 3 A and 4A , by kirchoff`s 1st law, the current flowing between x and B is 7A. now moving from A to x: Va-5*2+10=x, and then from x to B: x-7*1-5=Vb. solving these two equations we get Va - Vb= 12 V