Chapter 3 - Introduction to Graphing - 3.3 Graphing and Intercepts - 3.3 Exercise Set - Page 180: 77

$y=-1$

RECALL: A horizontal line's equation is of the form $y=k$. Its y-intercept is $(0, k)$, and each point on the line has a y-coordinate of $k$. The given graph is horizontal, and every point on the line has a y-coordinate of $-1$. This means the equation of the line is $y=-1$.