The setup for the first sphere in the cone is very similar to example problem 9.1 from the book.

With many 3D geometry problems, drawing a side view diagram can be helpful. Here is what the spheres look like inside the cone:

For the big sphere, we know the radius. What can be said about the right triangle formed by connecting the center of the sphere to the base of the cone (the orange line above)? 

For the small sphere, try drawing a similar line connecting the center of the big sphere to the base, this time going through the center of the small sphere.

Hope this helps, let us know if you have additional questions from here!