Answer:
2.37x10⁻⁷ M⁻².s⁻¹
Step-by-step explanation:
For a generic reversible reaction:
A + B ⇄ C + D
Kf is the constant of the formation of the products (C and D), Kr is the constant of the formation of the reactants (A and B), and Kc is the general equilibrium constant, which is:
Kc = Kf/Kr
2.76x10³ = 6.54x10⁻⁴/Kr
Kr = 6.54x10⁻⁴/2.76x10³
Kr = 2.37x10⁻⁷ M⁻².s⁻¹