Chapter 1 - Graphs - 1.3 Lines - 1.3 Assess Your Understanding - Page 31: 71

$y=4$

The line we are trying to find is perpendicular to the line $x=8$ Since $x=8$ is a vertical line, then the line perpendicualr to it is a horizontal line. The equation of a horizontal line is of the form $y=a$ where $a$ is a real number. All points on a horizontal line have the same $y$-coordinate. Thus, since the line is horizontal and contains the point $(3,4)$, the $y$-coordinate of which is $4$, then the equation of the line must be $y=4$.