If a more complicated atom has two energy levels for its electrons at (-15) eV and (-30.1) eV, what is the wavelength of the photon (in nm) that is emitted when an electron transitions between these two levels?

The energy difference between levels is 15.1 eV. Convert that to Ephoton, in Joules.

Ephoton = 2.42*10^-18 J

Set that equal to h*c/(wavelength) and solve for the wavelength.

h is Planck's constant.
c is the speed of light

The photon will be in the very far "vacuum" ultraviolet.