The frequency of a photon emitted by a hydrogen atom when it passes from the state n to the state n' is:

The base energy of the Hidrogen atom is -13.6eV and the Planck's constant is equal to:

Replace E_0=-13.6eV, n=5 and n'=4 to find the frequency of the emitted photon:

Therefore, the frequency of the emitted photon is approximately 3.40*10^13Hz