A certain amount of solid NH4HS is placed in a flask already containing ammonia gas at a certain temperature at 0.50 atm pressure. Ammonium hydrogen sulphide decomposes to yield NH3 and H2S gases in the flask. When the decomposition reaction reaches to equilibrium, the total pressure in the flask rises to 0.84 atm? The equilibrium constant for NH4HS  decomposition at this temperature is