The formula for the most stable form of oxygen is O₂, so the subscript is 2.
However, there is a less stable form of oxygen called ozone. Its formula is O₃, so the subscript for ozone is 3.
According to the Law of Conservation of Matter, the mass of the products in a chemical reaction must equal the mass of the reactants.
5O2 = 10 Oxygen atoms
In 2POx there must be 10 Oxygen atoms too. => x = 5
Hope it can help you.