#### Answer

A circle in the xy-plane of radius $9$ centered at the origin.

#### Work Step by Step

We put
$$ x=9\cos t, \quad y=9\sin t $$
hence we get
$$ x^2+y^2=81\sin^2t+81\cos^2t=81.$$
Which is a circle in the xy-plane of radius $9$ centered at the origin.