Voltage reference using zener diode and transistor
The antiquated MC3401 is not a normal opamp. It is actually of a type called a Norton amplifier and cannot be replaced by any modern types.

I would not recommend using this circuit even though the equation is correct.

There are many better solutions available now.

Here is an image of one section of the internal circuitry from RadioMuseum.Org.

It has a single common emitter amplifier stage with an output buffer.

The additional transistor Q3 and diode CR1 means that it responds to the difference between the input CURRENTs rather than the input VOLTAGES as any current injected in Vin(-) will be counteracted by the current from the current mirror Q3/CR1.