If the solubility of lithium sodium hexafluoroaluminate $\mathrm{Li}_{3} \mathrm{Na}_{3}\left(\mathrm{~A} / \mathrm{F}_{6}\right)_{2}$ is "a" mol/litre, its solubility product is equal to(1) $\mathrm{a}^{2}$
(2) $12 a^{2}$
(3) $18 a^{3}$
(4) $2916 \mathrm{a}^{8}$