For the circuit above, in addition to what's labeled, the Zener voltage is given as Vz = 4.7V, and we have that 11.5V < Vin < 13.5V. Now, as long as the op amp is not saturated, and the Zener diode is ideal, then V+ = V- = Vz, so that Vout = Vz(R3 + R4)/R4 = Vz(1 + R3/R4).
But then they ask, what is the maximum output power once a load RL is added? Okay, but Vout apparently has no dependence whatsoever on RL; it has already been fixed by the Zener voltage in conjunction with the voltage divider. Then the output power is just Vout^2 / RL, so the output power is unbounded as RL approaches zero.
Am I missing something? Especially because I don't see how to incorporate the transistor here -- I believe it's a safety device that siphons current away from the output to prevent overload, but how does it enter the mathematical analysis? And later they ask for the variation of the collector current, so surely it enters somehow.