Q:  The Andromeda Galaxy, M31, is roughly 750 kpc from the Earth.
          What is the "lookback time" to this galaxy?

                                  3.08 x 10^(16) m
            D  =  750,000 pc  *  -------------------
                                       pc

               =  2.3 x 10^(22)  meters


          So the time it takes light to travel from M31 to us is

                     D       2.3 x 10^(22) m
            t  =   -----  =  ----------------  =  7.7 x 10^(13) s
                     c        3 x 10^(8) m/s
          

               =   2.4 million years