Microwave ovens emit microwave energy with a wavelength of 12.3 cm. What is the energy of exactly one photon of this microwave radiation?

The energy of a photon is proportional to the speed of light and inversely proportional to the wavelength. The constant of proportionality is Planck's constant, 6.626 * 10^-34 Js

E = hc / {wavelength}

E = [ 6.626 * 10^-34 Js * 2.997 * 10^8 m/s ] / 0.124 m

E = 1.60 * 10^-24 J