Write the formula for the polyatomic ion of ba3(po4)2: express your answer as an ion. express single charges as [+], not [1+]

Ba₃(PO₄)₂ is a compound made of a metal cation and polyatomic ion. 
polyatomic ion is when 2 or more atoms are covalently bonded together and together have a charge and can act as one compound that can bond with another atom. 
In this case Ba²⁺ is the metal cation and PO₄³⁻ is the polyatomic ion. this is called the phosphate ion. one phosphate atom is bonded to 4 O atoms with an net charge of -3 and acts as a single unit.

It was ernest rutherford