Answer:
x^2 + y^2 = 81
One form of writing an equation of a circle is:
(x-h)^2 + (y-k)^2 = r^2
In this form h is the x-coordinate of the center. For this problem h = 0, because the center is at the origin.
In this form k is the y-coordinate of the center. For this problem k = 0.
In this form r is the radius. For this problem r = 9
x^2 + y^2 = 81
