Here's why:

* Calcium (Ca): Calcium is a Group 2 element, meaning it forms a +2 cation (Ca²⁺).

* Hydrogen Sulfide (HS⁻): Hydrogen sulfide is a weak acid that forms the HS⁻ anion.

* Balancing Charges: To balance the charges, you need two HS⁻ anions to match the +2 charge of the Ca²⁺ cation.

Therefore, the formula for calcium hydrogen sulfide is Ca(HS)₂.