There are 3 chlorine on product side and only 2 chlorine on reactant side. Thus multiply chloriine on reactant side by 3 and chlorine on product side by 2 to make them equal.
Al+3Cl2→2AlCl3
Now, there are two Al atom on product side but only one Al atom on reactant side. Thus, muliplyAl by 2 on reactant side.