Hello I have been reading a textbook on cosmology and have come up against a self test question I just can't crack! Any help would be greatly appreciated: "The Cosmological redshift of a quasar is normally deduced from its emission lines. Let this redshift be ze. Suppose the quasar has ejected a cloud of gas at velocity v with respect to the quasar. We detect the gas as absorption at redshift za. Show that the velocity of ejection is given by: v/c = [ (1+ze)^2 - (1+za)^2 ] / [ (1+ze)^2 + (1+za)^2 ] The "e" and "a" should be subscript, and the ^2 should be superscript (squared). Thanks very much.