For the reaction 2H2 + O2 -- 2H2O, how do I give an equation for the equilibrium constant in terms of the concentrations of products and reactants involved?

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]).