The resonant frequency will be that for which R2 = XL(XC - XL).
For the circuit parameters stated, this will be satisfied when f = 78 kHz.
Answer for 4.37
22 mA \/ 0 deg.
Answer for 4.48
VTH = 10 V at the appropriate frequency for both a) and b).
a). ZTH = (500 + j10) ohm, slightly inductive.
b). ZTH = (500 - j10) ohm, slightly capacitive.
Answer for 6.7
The resonant frequency is 4.9 rad/sec.