We have to form the salt ammonium sulfate. The formula of the ammonium ion is NH₄⁺ and the formula of the sulfate ion is SO₄²⁻.
Ammonium cation has one positive charge and the sulfate ion has two negative charges. To make a neutral compound we will have to combine two ammonium ions with one sulfate.
Answer: the formula of ammonium sulfate is (NH₄)₂SO₄