Here's how to determine the formula:

* Iron(II) indicates that iron has a +2 charge (Fe²⁺).

* Phosphide indicates the presence of the phosphide ion, which has a -3 charge (P³⁻).

To balance the charges, we need three iron ions (3 x +2 = +6) and two phosphide ions (2 x -3 = -6). This gives us the formula Fe₃P₂.