Answer:
+3
Step-by-step explanation:
This is ionic bond in combination with the basic rules of the coordination number of atoms in molecules, the O atom valence electrons 6, 3 valence electrons, Al₂O3 need to fill 2 electronic outer orbit, Al lose 3 electrons also brings inner shell stability, LCM is 6, 2 Al provides 2 x3 electronics, then it must have 3 O to accept it. If you have 2 O, there's no stable home for the extra electrons. So it usually forms Al₂O3