We have, a circle C which touches the y− axis at (0,6) and cuts off an intercept 65 on the x− axis.
We know that, length of the chord of the circle using perpendicular distance from the center is 2r2−d2 where r is the radius of the circle and d is the perpendicular distance from the center to the chord.
From the given data, we have the following diagram

Hence, length of the chord of the circle ⇒2r2−d2=65
⇒r2−d2=35
On squaring on both sides
⇒r2−d2=45
⇒d2=r2−45
Given that circle touches the y−axis at (0,6). Therefore, d=6.
⇒r2=45−36
r=36+45
r=9.