Answer
Please see below.
Work Step by Step
$$r=a\sec \theta = \frac{a}{\cos \theta } \quad \Rightarrow \quad r\cos \theta = a \quad \Rightarrow \quad x=a$$This is the equation of a vertical line $a$ units to the right of the $y$-axis if $a \gt 0$ and $|a|$ units to the left of the $y$-axis if $a \lt 0$.
