Both of these elements have a charge based on the number of valence electrons. For example: Nitrogen has 3 more electrons (negative) than protons (positive) so it has a negative 3 charge.
Aluminum has a positive 3 charge (three more protons than electrons)
Oxygen has a negative 2 charge (two more electrons than protons)
In order to put these two elements together, they have to balance each other out so that way their compound has no charge.
So for example if you are combining Nitrogen with Hydrogen, you need to balance the charges. So what you do is since Nitrogen has a negative 3 and Hydrogen has a positive 1 you need three hydrogens (H3) to balance out one Nitrogen. so the compound would be H3N.
With this compound you need to have 2 Aluminum (which makes a positive 6 charge) and three Oxygen (which makes a negative 6 charge). When you combine them there is no longer a charge.
Basically think of it as simple addition where you're just trying to make 0. So 6+(-6)=0. This makes no charge.
I hope this kinda helped but please ask questions if you need any more help :)