H4P2O6 i suppose :)
Possible P2O6 MeansNot: Phosphate because "anhydrous phophate" is P2O5 (most close to the original Q.) and "Phoshate" is (in chemistry) PO43-
Most chemical compound names are taken from what and how many atoms that they are made of. For instance, diphosphorus decoxide is di- (2) phosphorus dec- (10) oxide, or, P2O10.
it starts with a metal. look at your notes.