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