Answer
15.3 V.
Work Step by Step
Closing S1 while S2 is open connects the RL combination in series with the emf source.
Apply Kirchhoff’s loop rule: $\epsilon-iR-L\frac{di}{dt}=0$.
$$iR=v_{ab}=\epsilon -L\frac{di}{dt}=18.0V-(0.380H)(7.20A/s)=15.3V$$