Voltage difference of circuit segment

[Rabbittt]

Homework Statement

There is a segment of a circuit with voltage sources and resistors. I need to find the difference in voltage between the ends. A picture of the circuit is attached.

If a current I=3 A exists between points a and b, R1=2 [PLAIN]http://lon-capa.mines.edu/adm/jsMath/fonts/cmr10/alpha/100/char0A.png, R2=8 [PLAIN]http://lon-capa.mines.edu/adm/jsMath/fonts/cmr10/alpha/100/char0A.png, V1=4 V, and V2=15 V,the voltage difference Vb
-Va is?

Homework Equations

V=IR

The Attempt at a Solution

My intuition told me that the voltage sources would not affect the voltage difference if there was a constant current. So I combined the resistors and applied Ohm's law. V=(3)(8+2). However this is not correct . Can someone help me with understand how the voltage sources in the circuit affect the circuit.

 

[gneill]

Voltage sources always enforce their potential differences across themselves regardless of what current is flowing.

You have four components in series: two resistors and two voltage sources. The resistors present potential changes according to the current flowing through them via Ohm's Law, while the voltage sources present their defined potentials in the directions indicated.