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