Given these initial concentrations: [SCN-] = 0.000646 M, [Fe₃+] = 0.000506 M, and [Fe(SCN)₂] = 0.000000 M, determine the value for k if the [Fe(SCN)₂] at equilibrium is 0.0000856 M.
a) 1.68 x 10⁶
b) 2.68 x 10⁵
c) 4.68 x 10⁶
d) 3.68 x 10⁵