Areaction has δg° = + 21.5 kj/mol, δh° = + 25.0 kj/mol, and δs° = + 15.0 j/mol•k can become spontaneous at a temperature of k.

Answer is:  can become spontaneous at a temperature of 1670 K.
ΔG = 21.5 kJ/mol.
ΔH = 25 kJ/mol.
ΔS = 15.0 J/mol·K = 0.015 kJ/mol·K.
T = ?
The change in Gibbs free energy (ΔG), at constant temperature and pressure, is: ΔG=ΔH−TΔS.
ΔH is the change in enthalpy.
ΔS is change in entropy.
T is temperature of the system.
When ΔG is negative, a reaction (occurs without the addition of external energy) will be spontaneous (exergonic).
TΔS > ΔH.
T = ΔH ÷ ΔS.
T = 25.0 kJ/mol ÷ 0.015 J/K·mol.
T = 1670 K.