Microwave ovens emit microwave energy with a wavelength of 12.8 cm. what is the energy of exactly one photon of this microwave radiation?

What you can do is apply the next formula:  Frequency  = Wavespeed* wavelenght. We need to take into account that the wavesepped for all the electromagnetic waves is 3x10^8 m/s . with that you will find the frequency. The formula will be Frequency= 3x10^8 m/s * 12.8 cm. As soon as you have the frequency then you apply another formula that is Energy (E) = planck's constant(h) * Frequency you found(f) and that's it. WARNING: fisrt you need to turn the wavelenght from cm to meters 

If there is no loss of kinetic energy in the collision, it is elastic.