when ΔG° is the change in Gibbs free energy
So according to ΔG° formula:
ΔG° = - R*T*(㏑K)
here when K = [NH3]^2/[N2][H2]^3 = Kc
and Kc = 9
and when T is the temperature in Kelvin = 350 + 273 = 623 K
and R is the universal gas constant = 8.314 1/mol.K
So by substitution in ΔG° formula:
∴ ΔG° = - 8.314 1/ mol.K * 623 K *㏑(9)
= - 4536