A circle centre C and diameter RO of length 13cm is drawn and passes through O and B.

What is the gradient of the line RO?

My scanner isn't working so i've attached a (really bad) sketch of what the diagram looks like, i didn't draw the circle because my paint skills ain't up to that much!

*I have been trying for ages to get this answer but my mind just goes blank everytime i look at it. I don't have any idea of what formula/method to use so just a nudge in the right direction would help me a lot thanks for any help you can give*