Question 39797
Yes the equatin is correct.  But the point (h, k) is the center of the circle.  Here that is (0. 0).
But since the point (3, -6) lies on the circle, its distance from the origin is the radius of the circle.
Its distance is found by
d^2 = 3^2 + (-6)^2
d^2 = 9 + 36 
d^2 = 45
and since d^2 is really r^2, we have
x^2 + y^2 = 45