In Fig. 26.17, consider the circuit sketched. The two batteries have negligible internal resistance and emf's ?1 = 28.0 V and ?2 = 42.0 V. The three resistors have resistances R 1 = 2.00 ?, R 2 = 5.00 ?, and R 3 = 1.00 ?. Calculate the potential difference V a - V b between points a and b.

Figure 26.17 R E1 b a R, E2 R