Answer
$K_{c} = \frac{[N_{2}O][NO_{2}]}{[NO]^3}$
Work Step by Step
The expression for Kc is the concentration of products raised to their respective stoichiometric coefficients divided by the concentration of the reactants raised to their stoichiometric coefficients.
For the reaction:
3 NO N2O + NO2
we would have
$K_{c} = \frac{[N_{2}O][NO_{2}]}{[NO]^3}$