How to Calculate the Mass of the Galaxies

i looked at a pair of spiral galaxies is in the process of colliding. We know they are because we can detect, when we look more closely, the red shift of the stars. This reveals the direction in which parts of those galaxies are moving. And so we can also determine, by the same methods, how massive the galaxies are. With spiral galaxies, they are literally spinning. And the faster they're spinning, the more massive the galaxies were.