Calculating Potential Difference Between Points A and B

[mujadeo]

Homework Statement

a current of 2A is flowing from a to b.
what is the potential difference between the points?

______a______3ohm_____-20V+_____1ohm_______+6V-________b_____


hopefully my diagram makes sense.

Homework Equations

this is a really simple question and i can't answer it because I am not understanding the whole concept.
Does current (or anything) stay constant in a cicuit??
do i follow the current round/thru circuit and do calcs along the way?
so voltage after 3ohm-R = IR = 6V
so 6V into 20V battery. now I am stuck. it goes into neg term first so is that -20V?? please help somene! thanks

The Attempt at a Solution

 

[rl.bhat]

If you assume a battery of EMF E volt is connected across the a and b such that a end is +ve and b end is -ve. Apply loop law, and find E. If you get E +ve, signs of a and b correct. If you get E -ve, reverse the signs of a and b.