Question

Consider this equilibrium constant expression: K= (H2S)(OH-)/(HS-) What does this expression represent?

Answer 1

Since OH- is in the expression we're looking for Kb (which is for finding the pOH out).

And since K's are in relation to the reactant (which is in the denominator below $\frac{x}{y}$ ), the solution is Kb for HS-

Answer 2

Products over reactants: 

HS- + H2O ----> H2S + OH- 

The expression represents Kb for HS–