Here is a hint for you!
Let the radius of the sphere be r.
Since VD = 8cm, by the pythagoras theorem, VO is . Since OC is the radius, it has also length r.
Now VC = VO + OC = 12 cm. Substituting, you'll get
Move the r over, square both sides, and solve accordingly.
I did this quickly, so you may want to check the working for the equations.