calculate the value of the equilibrium constant K for this reaction A+B <--> C from the following information

3C <--> D Kc= 7.1*10^-4
D <--> 3A + 3B Kc= 2.9*10^-2

3C ==> D k1 = 7.1E-4

D ==> 3A + 3B k2 = 2.9E-2
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3C ==> 3A + 3B k3 = k1*k2
C ==> A + B k4 = cube root(k3) and this is the reverse of the equation you want so
A + B ==> C k5 = (1/k4)