Chapter 1 - Graphs - 1.3 Lines - 1.3 Assess Your Understanding - Page 31: 72

$x=3$

The line we are trying to find is perpendicular to the line $y=8$ The line $y=8$ is a horizontal line. The line vertical. that is perpendicular to a horizontal line is vertical. A vertical line has an equation of the form $x=a$ where $a$ is a real number. All point on a vertical lline have the same $x$-coordinate. Since the line is vertical and contains the point $(3,4)$, whose $x$-coordinate is $3$, then the equation of the line must be $x=3$.