Put the first mass at the origin , put mass at the coordinate .
Lets apply definition eqn.
Note that the y and z coordinates of the center of mass are zero, the only non-trivial one is the x coordinate. So in the x-direction Note that if we made much heavier than than the center mass would be shifted to the origin. If the converse where true, the center of mass would be shifted towards . When they're equal, the center of mass lies right in the middle between the objects.