PLEASE HELP ME WITH NUMBER 3

 4Al  + 3O₂  →  2Al₂O₃

Explanation:

The problem here is in balancing the equation below:

     Al  + O₂  →  Al₂O₃

In balancing a chemical equation, the number of atoms on both sides of the expression must be the same;

   put the alphabets a, b and c before each specie;

                  aAl  + bO₂  →  cAl₂O₃

Conserving Al;

                    a  = 2c

Conserving  O:

                     2b  = 3c

  Now, let c  = 1, a  = 2

                b  =  

Multiply through by 2;

           c  = 2, a  = 4 and b  = 3

          4Al  + 3O₂  →  2Al₂O₃