Since the circles are concentric, R is the radius of bigger disc and R3 is the radius of the smaller disc. Therefore, in order to locate the centre of mass from the main axis, we take it as R−R3. About the concentric part, there exist two circles which have a common area along the main axis (passing through the centers of both the circles).Therefore, we locate the centre of mass of the whole system by reducing it to R−R3.