electrical homework help
If we apply the first law of Kirchhoff we can tell that the current in both resistors (in both circuits) is the same, as you said. Thus, the voltage fall on the 10 ohm resistors is 1V and the other is 2V. Now, you have to apply the second law to determine V between A and B.  

We need to add al voltages (including the source) and equal them to 0. The only thing to have in mind is the "direction" of that fall. In the first, is is against the direction so you'll need to substract, whereas in the second you'll need to add.

Vab1 = +1.5 V

Vab2 = -7.5 V