Hint for 12.4
The power gain G is given by the ratio of the output power to the input power. Here the input power will be given by V2il / Ril.
Hint for S12.1
The Thevenin generator to use in place of the Wheatstone bridge consists of VTH = 0.037 V and RTH = 1.0 k.
Later in the circuit analysis, you should find that the capacitive reactance at the characteristic frequency is 6.0k. At the operating frequency the capacitive reactance will thus be 2.0k.