Here's how to figure that out:

* Aluminum (Al): Aluminum has a +3 charge.

* Sulfate (SO₄): Sulfate is a polyatomic ion with a -2 charge.

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

This gives you the formula Al₂(SO₄)₃.