A mixture of Fe^3+ and SCN^- was allowed to come to equilibrium with FeSCN^2+. The concentrations of each were then determined as follows: [Fe^3+]= 6.8 x 10 ^-4 M, [SCN ^-]= 3.3 x 10 ^-4 M, [FeSCN ^2+]= 3.0 X 10 ^-5 M. Determine the equilibrium constant for this reaction.