The frustum has: radii of the top circle base is 9. the radii of the bottom circle is 12. The slant height is 6.

I know you draw the frustum into a cone by extending it. Then you do the volume of the cone which is 1/3 bh. I can't seem to figure out the height though. So I can't figure how to get the volume. I was planning to subtract the volume of the small cone (the part that was extended) from the overall cone...and I was going to do Pythagorean theorem to find the height. But it doesn't make sense to me.

if I am remembering correctly there is a theorem that says something about the slant height of the frustum to be half of the slant height of the whole cone. I did Pythagorean theorem but the numbers just totally canceled. This is the source of my problem. Illustrating: (12^2+h^2=12^2) <---------that was what I tried to do to find the height of the cone. Anyways please help me find the volume of the frustum.

Thank you so much for any help provided