Here's how to figure it out:
* Copper (II) tells us that copper has a +2 charge (Cu2+).
* Phosphide is the anion of phosphorus, which has a -3 charge (P3-).
* To balance the charges, we need three copper ions (3 x +2 = +6) and two phosphide ions (2 x -3 = -6).
This gives us the formula Cu3P2.
