... These pair of absorption lines will appear in all the spectra and they should be your principle source for determining the redshift. The rest wavelengths of these lines are 3933 angstroms and 3968 angstrons. A radial velocity of a galaxy is determined from the redshift as follows: . ... where c = speed of light = 300,000 km/s . ... We know the rest wavelength is 4861 and therefore the velocity of the galaxy (its redshift, since the line is shifted to longer wavelengths) would be: . ...