How does this voltage-reference circuit works
For a DC analysis you can ignore R4 and C1 since an ideal op-amp has no bias current and the capacitor is not important at DC.
You should be able to calculate the output voltage assuming that the zener is conducting, and then calculate the zener current. Assume the op-amp drives its inputs to be equal as hinted in the comments.
This circuit  has a potential problem. With a bipolar supply it has two stable states. With a supply as shown it also may  have two stable states depending on a few non-ideal things.