You just use the morality of the solution, you found out that you have 0.0046mol (about 0.005mol0 of AlCl3, so then since there is 1 Al in a single molecule, you have the number of moles of aluminum, but since there is Cl3 (3 atoms of chlorine per molecule), just multiply the number of moles of molecules by the number of atoms of that type inside a single molecule, to get [Cl-] = 0.0138mol (or round to 0.014)