Chapter F - Foundations: A Prelude to Functions - Section F.3 Lines - F.3 Assess Your Understanding - Page 30: 63

$x=4$

For a vertical line, the slope is undefined and the line's equation comes in the form $x=a$, where $a$ is a constant. The line $x=5$ is vertical. Lines that are parallel to a vertical line are also vertical lines. Since the line parallel to $x=5$ contains the point $(4, 2)$ whose $x$-coordinate is $4$, then the equation of the line must be $x=4$.