The equilibrium-constant of the reaction no2 (g) + no3 (g) ⇌n2o5 (g) is k=2.1*10-20. what can be said about this reaction?

The equilibrium constant is a ratio of the concentration of the products raised to their respective stoichiometric coefficients to the concentration of the reactants raised to their respective stoichiometric coefficients. For this reaction, K is equal to

K = [N₂O₅]/[NO₂][NO₃]

So, if K is greater than 1, the reaction would favor the product side. Otherwise, it would favor the reactant side. Since the K here is less than 1 as manifested by the negative exponent of 10, the reaction would favor or would be spontaneous for the forward reaction.