#### Answer

$y$

#### Work Step by Step

RECALL:
A horizontal line has an equation in the form $y=k$ where $k$ is a real number.
All points on the line have a y-coordinate of $k$.
The given horizontal line contains the point $(-4, 8)$.
The y-coordinate of this point is $8$.
Thus, the equation of the horizontal line is $y=8$.
Therefore, the missing expression in the given statement is:
$y$