Here's how we arrive at this formula:

* Ferrous indicates the iron ion with a +2 charge (Fe²⁺).

* Phosphate refers to the polyatomic ion PO₄³⁻.

To balance the charges, we need three iron ions (3 x +2 = +6) and two phosphate ions (2 x -3 = -6). This gives us the final formula Fe₃(PO₄)₂.