Find Vo in an op-amp - transistor circuit
The trick to remember with linear op-amp circuits is that you can assume (without any serious error) that V+ and V- inputs are at the same potential. You can also assume that the resistor in series with V+ input has no effect on reducing the signal into the input V+.

With this in mind the voltage across R2 has to be 4V and the voltage on the emitter of Q1 has to be 12V. This is the answer, it's as simple as that.

To help you understand a bit more, the transistor Q1 plays no-part in making the voltage output bigger or smaller. The circuit could have been redrawn with OA1's output straight to where the emitter is and the transistor lost completely. This works exactly the same.

The reason some circuits build a BJT in is that it can provide more current drive down to 0V for when certain output loads are connected.