Here's how to figure that out:

* Iron (III) indicates the iron ion has a +3 charge (Fe³⁺).

* Sulfate is a polyatomic ion with a -2 charge (SO₄²⁻).

* To balance the charges, you need two iron ions (2 x +3 = +6) and three sulfate ions (3 x -2 = -6).

Therefore, the formula is Fe₂(SO₄)₃.