The chemical formula of zinc phosphate is

The chemical formula of zinc phosphate is $Z{n}_3(P{O}_4{)}_2$.
Zinc phosphate contains three $Z{n^{2+}}_{ }$  and two $P{O}_4^{3-}$

As we know,
Charge on zinc  $=+2$
Charge on phosphate $=-3$
So, the charges over the ions get cross-multiplied to form a chemical formula.