For any reaction, the equilibrium constant is given in the form: K = [products] / [reactants]
Where [ ] is used to denote the concentration of the species.
Therefore we can substitute in for the molecules given above, which becomes, K = [H2O] / ([H2] x [O2]).
But, we're not finished there. We have to remember the stoichiometric coefficients given in the balanced equation, and put them to the power of the respective molecules.
This gives the final, correct answer, K = [H2O]2 / ([H2]2 x [O2]).