The oxidation number for Zn is +2
We know this by the fact that that it is known that oxygen almost always has an oxidation number of -2 while hydrogen almost always has an oxidation number of +1. (In both of those cases I said almost always because there are always exceptions to everything but you probably won't need to deal with the exceptions). When you add -2 and +1 you get -1 for the OH part of the molecule. Since there are 4 OH groups in the molecule they make a -4 charge. We know that the molecule has an overall charge of -2 which means that Zn needs a charge of +2.
I hope this helps. Let me know if you have any further questions or need anything clarified.