I stole most of this from the web.
From Pythagoras:
AE^2 + DE^2 = r^2
r^2 = (x+4)^2 ..............................(1)
r^2 = (12-x)^2+36
therefore
(x+4)^2 = (12-x)^2+36
simplify and calculate
x= 5
therefore
r = sqrt(9^2 + 4) (from (1))
= sqrt(85)
Cool solution huh?