$y=3$ Refer to the graph below.
Work Step by Step
RECALL: A horizontal line has an equation of the form $y=k$. Every point on the line has a y-coordinate $k$. The line is horizontal so its equation is of the form $y=k$. The line contains the point $(2, 3)$. The y-coordinate of this point is $3$. Since all points on a horizontal line have the same y-coordinate, then the equation of the line is $y=3$. Refer to the graph in the answer part above.