Use the sum of squares of two right angles of a right triangle = the square of the hypotenuse.

So the square of x =140 *140+(x-20) * (x-20).

X * X = 19600+X * X+20 * 20-2 * X * 20

X*X=20000+X*X-40X

40X=20000

X=500

So the radius of the circle is 500 meters.