Answer
See below.
Work Step by Step
Use the relation $r\ cos\theta=x $, we have $r\ cos\theta=a\Longrightarrow x=a$ in rectangular coordinates.
This means that, when $a\ge0$, it is a vertical line $a$ units to the right of the pole; when $a\lt0$, it is a vertial line $|a|$ units to the left of the pole.
