Here's why:

* Antimony (Sb): Antimony is a metalloid with a variable oxidation state. In this case, it's in the +3 oxidation state, indicated by the Roman numeral III.

* Oxygen (O): Oxygen typically has an oxidation state of -2.

To balance the charges, you need two antimony atoms (+3 each) and three oxygen atoms (-2 each) resulting in a net charge of zero:

* 2 (+3) + 3 (-2) = 0