Equation for the decomposition of ammonia :
2NH3->N2+3H2
Equilibrium constant for this equation will be:
Kc=[N2]([H2]^3)/[NH3]^2
As given Concentration of N2=0.800 M
Concentration of H2=0.420 M
Concentration of NH3=0.250 M
So Kc=([0.420]^3) *(0.8)/([0.250]^2)
= 0.9483264
so equilibrium constant will be:
0.9483264