Start with the equation of a circle whose center is at (h,k) and whose radius is r:
(x-h)^2 + (y-k)^2 = r^2
Substituting the given coordinates of the center:
(x-5)^2 + (y-[-5])^2 = r^2, or (x-5)^2 + (y+5)^2 = r^2
Substituding the given coordinates of a point (6,-2) on the circle:
(6-5)^2 + (-2+5)^2 = r^2
Simplifying:
1^2 + 3^2 = r^2, or 1 + 9 = r^2, or 10= r^2. Then r = sqrt(10).
