The correct expression for $K_{eq}$ for the reaction below is $2N{H}_3(g)longleftrightarrow{N}_2(g)+3H_2(g)K_{eq}=\frac{[N_2]}{[H_2]}{K}_{eq}=\frac{[N{H}_3{]}^2}{[N_2][H_2{]}^3}{K}_{eq}=\frac{[N_2][H_2{]}^3}{[N{H}_3{]}^2}{K}_{eq}=\frac{[N_2]+[H_2{]}^3}{[N{H}_3{]}^2}.$