Please teach me how to do part （a）?

r^2=OA^2=AM^2+OM^2=OC^2=CN^2+ON^2
AM^2+OM^2=CN^2+ON^2
AB=CD ---> AM=CN ---> ON=OM ....Ans
20^2=(32/2)^2+ON^2 ---> ON=OM=12 ...Ans