The formula for antimony(III) phosphide is SbP. Here's how we arrive at that:

* Antimony(III) indicates that antimony (Sb) has a +3 oxidation state.

* Phosphide indicates that phosphorus (P) has a -3 oxidation state.

Since the charges are equal and opposite, the simplest ratio of antimony to phosphorus is 1:1. This results in the formula SbP.