I tried this way:
First I find the distance between the 2 center of sphere. Then I substract it with the radii of both sphere. Am I right?
I get the answer [tex]3\sqrt{3}-3[\MATH]
Find minimum distance between any point on the sphere and any point on the sphere .
I find the distance between the two center,in which is [tex]3\sqrt{3}[\MATH]. Then I substract the distance between 2 center by both the radii of the sphere, that is, I get the minimum distance to be [tex]3\sqrt{3}-1-2=3\sqrt{3}-3[\MATH].