Here's how to figure that out:
* Iron(III) indicates that the iron ion has a +3 charge (Fe³⁺).
* Selenide refers to the selenium anion, which has a -2 charge (Se²⁻).
* To balance the charges, we need two iron ions (2 x +3 = +6) and three selenium ions (3 x -2 = -6).
Therefore, the formula is Fe₂Se₃.
