First, we write our reaction:
Al + Cl2 ==> AlCl3
To balance a reaction similar to this one, we must focus on having the same number of atoms on each side, I mean on the left side and on the right side of the reaction.
As we can see, we have 2 Cl on the left and 3 on the right, therefore we try to have the same number of this atom, multiplying by a pair number and an odd number respectively e.g. by 3 and 2.
Al + 3 Cl2 ==> 2 AlCl3
After that, we have 6 Cl on both sides. Then, we must go on with Al, and the only thing we should do is multiply on the left by 2 because we already have 2 Al on the right side.
2 Al + 3 Cl2 ==> 2 AlCl3
With this, we will have 2 Al and 6 Cl in total
Answer: 2 Al + 3 Cl2 ==> 2 AlCl3