I had a student ask me this question. I have never seen two sources connected in parallel quite like this so I thought I'd see if anyone with more knowledge about circuitry could verify my thoughts.
The parallel combination of sources sets up a short-circuit in the system, with a "back" current balancing out to produce an overall 3 V potential across the resistor.
Obviously in reality the sources have internal resistance and the problem is easily solvable. It's the zero internal resistance case I am curious about.