The equilibrium constant, represented as a variable K, is the ratio of the concentration of the products to the reactants which are raised respectively to their stoichiometric coefficients. Note that only substances in the aqueous phases are accounted. From the reaction given, K is equal to
K = [Sn²⁺]³/[Fe³⁺]²
Once you know the equilibrium concentration of Sn²⁺ and Fe³⁺ in molarity, just substitute them into the equation to solve for K.
