Calculating Wavelength Using Atomic Emission Spectra and Balmer Series Equation

[apiwowar]

in an experiment we looked through the diffraction grating to see the spectrum made by a helium lamp.

we measured the distance along the meter stick to the point where we saw each color.

based on that distance how would you calculate the wavelength of the light that was seen?
would you use the balmer series equation? and if so what would be the principal quantum numbers?

any help on how to figure this out would be appreciated

 

[chemisttree]

See http://sunny.moorparkcollege.edu/~chemistry/Chemistry_1A_labs/experiment_nine.pdf"