#### Answer

$\color{blue}{x^2+y^2=81}$
Refer to the graph below.

#### Work Step by Step

RECALL:
The center-radius form of a circle's equation is $(x-h)^2+(y-k)^2=r^2$ with center at $(h, k)$ and a radius of $r$ units.
Thus, the given circle's equation in center-radius form is:
$(x-0)^2+(y-0)^2=9^2
\\\color{blue}{x^2+y^2=81}$
To graph the circle, perform the following steps:
(1) Plot the center.
(2) Plot the points 9 units above, below, to the left, and to the right of the center.
(3) Connect the four points in Step 2 using a curve to form a circle.
(Refer to the attached image in the answer part above for the graph.)