Answer :
[tex]x^2+y^2=9\Leftrightarrow (x-0)^2+(y-0)^2=3^2[/tex] hence the center is (0,0) and the radius is 3
[tex](x-x_1)^2+(y-y_1)^2=r^2\\
\text{center}=(x_1,y_1)\\\\
x^2+y^2=9\\
(x-0)^2+(y-0)^2=9\\
\text{center}=(0,0)\\
r=\sqrt9=3
[/tex]