#### Answer

center: $(0, 0)$;
radius = 3 units
Refer to the image below for the graph.

#### Work Step by Step

RECALL:
The circle $x^2+y^2=r^2$ has its center at $(0, 0)$ and a radius of $r$ units.
The given circle can be written as $x^2+y^2=3^2$.
This equation is in the same form as the one in the recall part above so its center is at $(0, 0)$ and its radius is $3$.
Plot the points that are:
3 units above the center: $(0, 3)$
3 units below the center: $(0, -3)$
3 units to the left of the center: $(-3, 0)$
3 units to the right of the center: $(3, 0)$
Then, connect the points using a curve to form a circle.
(refer to the attached image in the answer part above)