Here's how to arrive at that formula:

* Nickel(II) means the nickel ion has a +2 charge (Ni²⁺).

* Arsenate is the polyatomic ion with a -3 charge (AsO₄³⁻).

* To balance the charges, you need three nickel ions (3 x +2 = +6) and two arsenate ions (2 x -3 = -6).

This gives you the formula Ni₃(AsO₄)₂.