Hydrogen atoms are excited by a laser to the n = 4 state and then allowed to emit.

Hydrogen atoms are excited by a laser to the n = 4 state and then allowed to emit. What is the maximum number of distinct emission spectral lines (lines of different wavelengths) that can be observed from this system?

which is six but i cant get the second part which is

but how can i calculate the wavelength of the 2 to 1 transition