A 1.00L flash was filled with 2.00mol gaseous SO2 and 2.00 mol gaseous NO2 and heated. After equilibrium was reached, it was found that 1.30 mol gaseous NO was present. The reaction is:

SO2(g) + NO2(g) <---> SO3(g) + NO(g)

Calculate the value of the equilibrium constant, K, for this reaction.

I thought perhaps it was some sort of RICE equation, and while it is easy to find the molatiry of the two reactants and NO(g) compouds, I cannot figure how to find the molarity of SO3 to calculate the constant. I also just thought that maybe, due to the reactants being equal, at equilibrum so would the products, thus making the molarity of SO3(g) 1.3, but I am not getting the answer my book provides, which is stated to be K=3.4.