Answer
$$K = 23 $$
Work Step by Step
1. Calculate $\Delta n$ (n is the amount of moles of gases):
$$\Delta n = n_{products} - n_{reactants} = 2 - 3 = -1 $$
2 . Calculate Kc:
$$K = \frac{K_p}{(RT)^{\Delta n}} = \frac{( 0.25 )}{(0.0821 \times 1100 )^{ -1 }}$$
$$K = 23 $$