1)  Vth:  solve circuit for voltage between upper right node and bottom node.  24 volts;
      Rth:  turn "off" independent voltage and current sources and calculate resistance looking into the port.  Rth = 5 ohm || 20 ohm  =  4 ohm

2) Negative feedback, so use ideal assumptions.  I1 = +0.2mA;  I2 = 0;  I3 = +0.2mA (= I1); I4 = -0.7 mA

3) Capacitor voltage starts at zero and cannot change instantaneously, so capacitor current must jump from zero to V/R.  So Figure c is the correct solution.

4) Initial inductor current = Vs/R1
    Inductor current at inf = 0
    Inductor current for  t>0:   (Vs/R1) x  exp{  -t /  (L/R)  }

5) Working from right to left, notice that voltage from top right node to bottom is 5V, then determine currents.  Result is Vx = 25 volts.

6) Note current source in upper mesh, so i1 = - 5 amps  (direction opposite i1 direction).  Then solving lower mesh gives i2 = 2 amps.

7) Efficiency is Pout/Pin  = 85%

8) Speed regulation is 5.3%

9) Vth:  no current in elements because Vth is "open circuit" voltage, so Vth = 20 at 0 degrees.  Zth is jwL + 1/(jwC) = 90 at -90 degrees.