Chapter 1 - Graphs - 1.3 Lines - 1.3 Assess Your Understanding - Page 31: 59

$y=2$

If the line is horizontal, then the equation must be in the form $y=k$ where $k$ is a real number. All points on the line have the same $y$-coordinate of $k$. The line is horizontal and contains the point $(-3,2)$, the $y$-coordinate of which is $2$. Hence, the equation of the line must be $y=2$.