Answer
See below.
Work Step by Step
Use the relation $r\ sin\theta=y $, we have $r\ sin\theta=a\Longrightarrow y=a$ in rectangular coordinates.
This means that, when $a\ge0$, it is a horizontal line $a$ units above the pole; when $a\lt0$, it is a horizontal line $|a|$ units below the pole.
