Here's how to figure that out:

* Ammonium is the cation (positively charged ion) formed from ammonia (NH₃) and has the formula NH₄⁺.

* Sulfite is the anion (negatively charged ion) derived from sulfurous acid (H₂SO₃) and has the formula SO₃²⁻.

To balance the charges, we need two ammonium ions (2+ charge) for every one sulfite ion (2- charge). This leads to the formula (NH₄)₂SO₃.