How do I find the empirical formula for aluminium selenide?

Question

You have to have some data, such as masses, or percent: Empirical means it was measured.

The empirical formula for aluminium selenide is the same as it's molecular formula; Al2Se3.
The empirical formula requires both or all atom quantities to be divided by the same number, so Al9Se3 would become Al3Se, but Al7Se3 would stay the same.