The energy of electron in the second and third Bohr orbit of the hydrogen atom is –5.42 x 10–12 erg and –2.41 x 10–12erg, respectively. Calculate the wavelengths of emitted radiation when the electron drops from third to second orbit.

for such transition related questions, energy of electron emitted is equal to energy difference of the orbitals between which the transition occurs. in this case, transition occurs between second and third orbit and energy of both is given. therefore the energy difference between these two = energy of electron emitted. and we know that energy of an emitted electron is related to wavelength as E=hc/lambda, lambda being the wavelength. go ahead and try the sum out, now you know the concept, let me know  if you're still unable to solve it, i’d be more than happy to do it for you :)