A definite amount of solid $N{H}_{4}HS$ is placed in a flask already containing ammonia gas at a certain temperature and 0.50 atm pressure. $N{H}_{4}HS$ decomposes to give $N{H}_3$ and $H_{2}S$ and at equilibrium total pressure in flask is 0.84 atm. The equilibrium constant for the reaction is $\left(at{m}^2\right)$: