The reaction that takes place when the solid NH4HS is heated is:
NH4HS(s) --> NH3(g) + H2S(g)
While estimating the equilibrium constant Kc only the concentration of the solid reagents is not taken into consideration.
For a generalized reaction: A(g) + B(g) --> C(g) + D(g), Kc = [C][D]/[A][B]
Here, [NH2] = 0.3*5.1 = 1.53 and [H2S] = 0.3*5.1 = 1.53
The equilibrium constant Kc = 1.53*1.53 = 2.3409
The equilibrium constant for the reaction NH4HS(s) --> NH3(g) + H2S(g) for the given conditions is Kc = 2.3409