The open-circuit terminal voltage of a three-phase, 60-Hz synchr

The open-circuit terminal voltage of a three-phase, 60-Hz synchronous generator is found to be 15.4 kV rms line-to-line when the field current is 420 A.
a. Calculate the stator-to-rotor mutual inductance Laf.
b. Calculate the open-circuit terminal voltage if the field current is held constant while the generator speed is reduced so that the frequency of the generated voltage is 50 Hz.