Write the equilibrium constant for this reaction in terms

Question

Consider the reaction:

1/2 N₂(g) + O₂(g) <-----> NO₂(g)

Write the equilibrium constant for this reaction in terms of the equilibrium constants, K_a and K_b, for reactions a and b below:

a.) N₂O₄(g) <----> 2NO₂(g)   K_a

b.) N₂(g) + 2 O₂(g) <------> N₂O₄(g)  K_b

K =________