Here's why:

* Aluminum (Al) is a metal in group 3 of the periodic table and has a +3 charge as an ion (Al³⁺).

* Bromine (Br) is a nonmetal in group 17 of the periodic table and has a -1 charge as an ion (Br⁻).

To balance the charges, you need three bromine ions (3 x -1 = -3) to match the +3 charge of the aluminum ion.