so its 707 to 0dp
You're overdoing things!
You need to practice with factors,
Be very accustomed to those.
Then you have Pythagoras' theorem to master.
Firstly, zero multiplied by anything is zero...
and so on...
So, from that, what possible values can we have for the radius ?
If the 2 givens (3 and 6 in your case) are represented by u and v, and d = diameter of circle, then:
d = [-b + SQRT(b^2 - 4c)] / 2, where b = -4(u + v) and c = 4(u^2 + v^2)
Stick 3 and 6 in there and you'll get d = 30 (hence radius = 15).
Try u=88, v=99: d = 638