Answer
See below.
Work Step by Step
Use the relations $r^2=x^2+y^2, r\ sin\theta=y $, we have $r=-2a\ sin\theta\Longrightarrow r^2=-2ar\ sin\theta\Longrightarrow x^2+y^2=-2ay\Longrightarrow x^2+(y+a)^2=a^2$, thus it is a circle with center $(0,-a)$, radius $r=a$ in rectangular coordiantes.
