Answer
Balanced chemical equation:
$4Al(s) + 3O_2(g) --\gt 2Al_2O_3(s)$
Work Step by Step
$Al(s) + O_2(g) --\gt Al_2O_3(s)$
1. Balance the number of oxygens, by putting a "2" in the front of $Al_2O_3$, and a "3" on $O_2$.
$Al(s) + 3O_2(g) --\gt 2Al_2O_3(s)$
2. Balance the number of aluminum atoms, by putting a "4" in the front of $Al$:
$4Al(s) + 3O_2(g) --\gt 2Al_2O_3(s)$
