According to Kohlrausch's law of independent migration of ions, the limiting equivalent conductance of an electrolyte is the algebraic sum of the limiting equivalent conductances of its constituent cations and anions. Therefore, for $Al_2(SO_4)_3$, the equivalent conductance at infinite dilution is $\Lambda^{\circ}_{eq} = \Lambda^{\circ}_{Al^{3+}} + \Lambda^{\circ}_{SO_4^{2-}}$. Note that if the question asked for molar conductance, it would be $\Lambda^{\circ}_{m} = 2\Lambda^{\circ}_{m}(Al^{3+}) + 3\Lambda^{\circ}_{m}(SO_4^{2-})$.