Answer
$\color{blue}{x=3}$
Work Step by Step
RECALL:
(1) A vertical line has an equation $x=h$ where $h$ is the x-intercept. Every point on this line has $x=h$.
(2) A horizontal line has an equation $y=k$ where $k$ is the y-intercept. Every point on this line has $y=k$.
(3) A vertical line and a horizontal line are perpendicular to each other.
The line we are looking for is perpendicular to the horizontal line $y=8$.
This means that the line we are looking for is vertical.
Since the line contains the point $(3, 4)$, then every point on the line has $x=3$.
Thus, the equation of the line we are looking for is:
$\color{blue}{x=3}$
