Use the definition of
$K_(eq) = (["products"]^(d,e,f, . . . ))/(["reactants"]^(a,b,c,. . . ))$
$= (["NH"_3]_(eq)^2)/(["H"_2]_(eq)^3["N"_2]_(eq))$
Just make sure you remember to use stoichiometric coefficients correctly.
Now, plug in your concentrations and evaluate. Are they or are they not already equilibrium concentrations?