The unbalanced chemical reaction is Al(s)+O2(g)→Al2O3(s)
Balance Al atoms by adding 2 to Al on LHS
2Al(s)+O2(g)→Al2O3(s)
Balance O atoms by adding 32 to O2 on LHS
2Al(s)+32O2(g)→Al2O3(s)
Multiply the whole equation with 2.
4Al(s)+3O2(g)→2Al2O3(s)
This is the balanced chemical equation. The coefficient of O2 is 3.