Due to a weak connection, there is a break in the circuit (shown below) between points a and b. If R1 = 2.23 Ω, R2 = 8.2 Ω, and R3 = 7.95 Ω, and the two batteries ε1 and ε2 have voltages of 8 V and 6.50 V respectively, find the potential difference Vab. Be sure to include the sign of the potential difference.