We take activities of Solids and liquids equal 1. The reason is there concentrations not at all change amount of reactant at equilibrium in the reaction. Hence we don't consider concentrations of pure solids and liquids while writing equilibrium constant.
So for above reaction, equilibrium constant (K) = [Sn^2 +] ^3 x 1 / [Fe^3+]^2 x 1. Ping me if you have any doubts.
