Microwave ovens emit microwave energy with a wavelength of 12.7 cm. what is the energy of exactly one photon of this microwave radiation?

Photon energy is the energy carried by a single photon with a certain wavelength and frequency. These terms are related by an equation. 
E = hv
where h is the Planck's constant (6.626 x 10-34 J s), v is the frequency which is related to the wavelength by:

λv = c where λ is the wavelength and c is the speed of light (3.00 x 108 m/s)

Therefore, the energy equation 
E = hc / λ
E = (6.626 x 10-34 J s)(3.00 x 10^8 m/s) / 0.127 m = 1.565 x 10^-24 J