2ICl <========> I2 + Cl2 Kc (equilibrium)
ICl (g) <==========> 1/2 I2 (g) + 1/2 Cl2 (g) K'c
If you multiply the first reaction by 1/2 you will get the second one.
Now, what happens with the Constants?
If you multiply by a number, the constant must be raised to the power of that number.
There are the same reactants and products, but we have different coefficients.
Then
[tex]K^{\prime}c\text{ = }\sqrt[]{Kc}[/tex]E.g. if Kc = 12
[tex]K^{\prime}c\text{ = }\sqrt[]{Kc}=\text{ }\sqrt[]{12}=\text{ 3.46 (approx.)}[/tex]