Answer
$x^2+y^2=9$
Work Step by Step
The standard form of a circle with center at $(h,k)$ and radius $r$ is:
$$(x-h)^2+(y-k)^2=r^2$$
The standard form of a circle with center at the origin ($h=0,k=0$) is:
$$x^2+y^2=r^2$$
Substituting $r=3$:
$$x^2+y^2=3^2$$ Simplifying:
$$x^2+y^2=9$$
