Here's how we arrive at that:

* Chlorine (Cl) is a halogen and has a charge of -1 when it forms an anion.

* Bromine (Br) is also a halogen and has a charge of -1 when it forms an anion.

* To balance the charges, we need one chlorine atom (Cl) and two bromine atoms (Br), resulting in the formula ClBr₂.