x^2 + y^2 = 9 The general equation for a circle of radius r centered at point (h,k) is (x - h)^2 + (y - k)^2 = r^2 Since we want it centered at the original, h and k are both 0, so the equation simplifies to: x^2 + y^2 = r^2 Substituting the known values for the given point gives: x^2 + y^2 = r^2 0^2 + 3^2 = r^2 0 + 9 = r^2 9 = r^2 3 = r
