Here's how to determine it:

* Rubidium is an alkali metal with a +1 charge (Rb⁺).

* Chromate is a polyatomic ion with a -2 charge (CrO₄²⁻).

To balance the charges, you need two rubidium ions (2 x +1 = +2) for every one chromate ion. This gives you the formula Rb₂CrO₄.