The first excited state of the hydrogen atom is separated from the ground state by 10.2 eV.
(a) At what wavelength does this occur and what is the energy difference in kJ mol-1
The energy difference is 10.2 eV can be convert to Joules by 1 eV = 1.602 x 10^-19 joules.
Then delta E = hc/wavelength.
You know delta E, h is Planck's constant, and c is the speed of light in m/s. Wavelength will have meters as the unit.