Appendix B - Coordinate Geometry and Lines - B Exercises - Page A15: 32

$x=4$

We are told to find the equation of the line that satisfies the following conditions: Parallel to $y$-axis and passes through (4,5) Since the line is parallel to the $y$-axis and $x=4$, $x$ will equal 4 for every y value. So the equation is $x=4$