Remember, the volume formula of a sphere is [tex] \frac{4}{3} \pi r^{3} [/tex].
Set that equation equal to whatever volume they give you and solve for r.
[tex] \frac{4}{3} \pi r^{3} [/tex] = [tex] \frac{9}{16} \pi[/tex]
Now solve for r! I know you can do this. If you need help, just comment.