Optics question. Given Diameter and object distance find image distance.

[mfreeman91]

An object is 9.79 cm from the surface of a reflective spherical Christmas-tree ornament 8.96 cm in diameter.
What is the position of the image? Answer in units of cm

I know that i need the radius of curvature to find the position of the image, but I'm not sure how to get that from the diameter? I tried just dividing by 2, but it says it is wrong. I just need help on finding the radius of curvature please, thanks!

 

[mfreeman91]

Nevermind!