#### Answer

$x^2+y^2=81$

#### Work Step by Step

$\bf{\text{Solution Outline:}}$
Use the Center-Radius Form of the equation of circles to get the equation with the given center, $C(
0,0
),$ and the given radius, $r=
9
.$
$\bf{\text{Solution Details:}}$
With the given center, then $h=
0
$ and $k=
0
.$ Using the Center-Radius Form of the equation of circles which is given by $(x-h)^2+(y-k)^2=r^2, $ the equation of the circle is
\begin{array}{l}\require{cancel}
(x-0)^2+(y-0)^2=(9)^2
\\\\
x^2+y^2=81
.\end{array}