Q:  What is the shift in wavelength between light emitted
       by the bottom edge of the galaxy and light emitted by
       the top edge of the galaxy?


          The H-alpha line from the bottom is at 664.0 nm
          The H-alpha line from the top    is at 665.0 nm
     
          That makes a shift of 1.0 nm from bottom to top.


       How fast is the galaxy rotating?


          The difference in radial velocity between the
          top and bottom of the galaxy is about

                              1 nm 
              velocity  =  ----------  * (300,000 km/s)
                             664.5 nm

                        =   450 km/s


          So we say that the galaxy is rotating at about
          225 km/s: one side comes towards us at that
          speed, and the other moves away from us that
          speed.