Compute all parts of given question:

Question: In a Franck-Hertz type of experiment, atomic hydrogen is bombarded with electrons and excitation potentials are found at 10.21 V and 12.10 V.

Part A: Explain the observation that three different lines of spectral emission accompany these excitations (Hint: Draw an energy-level diagram.)

Part B: Now assume that the energy differences can be expressed as hf and find the three allowed values of f.

Part C: Assume that f is the frequency of the emitted radiation and determine the wavelengths of the observed spectral lines.

Please help for the above question, completely confused!! Show each steps if possible, Thanks.