3和7的公倍数,两位数的有21,42,63,84
21的约数1,3,7,21,四个
42的约数,1,2,3,6,7,14,21,42,八个.
63的约数,1,3,7,9,21,63,六个,符合条件.
84的约数,1,2,3,4,6,7,12,14,21,42,84,十个
所以是63.