The figure shows two cells and a resistor joined in a single loop. The emf and internal resistance of each cell are marked, and P and Q are the terminals of cell 2, with P the terminal marked positive. The connecting wires have no resistance. What is the potential difference $V_P - V_Q$ between the terminals of cell 2?

Fig. 1: A single loop. Cell 1, of emf 9.0 V and internal resistance 1.0 ohm, is on the left with its positive terminal up. A resistor of 3.0 ohm is in the top wire. On the right, cell 2, of emf 3.0 V and internal resistance 2.0 ohm, also has its positive terminal up, so that on this side of the loop it opposes cell 1. A dashed box encloses cell 2 with its internal resistance. P and Q mark the top and bottom terminals of cell 2.