At 218 °C, Kc = 1.2 × 10⁻⁴ for equilibrium
NH₄SH(s) ⇌ NH₃(g) + H₂S(g)

Calculate the equilibrium concentration of NH₃ if a sample of solid NH₄SH is placed in a closed vessel at 218 °C and decomposes until equilibrium is reached.