Answer
$x=3$
Work Step by Step
A horizontal line has an equation in the form $y=a$, where $a$ is a constant.
Thus, the line $y=8$ is horizontal.
Recall that a line perpendicular to a horizontal line is a vertical line.
A vertical line has an equation of the form $x=a$ where $a$ is a constant.
Since the line we are looking for is perpendicular to $y=8$ (which is a horizontal line), then the line must be horizontal.
The line contains $(3, 4)$, whose $x$-coordinate is $3$, so the line's equation must be $x=3$.
