Chapter 1, Equations and Graphs - Section 1.3 - Lines - 1.3 Exercises - Page 113: 38

$y=1$

RECALL: A horizontal line has a slope of zero and has an equation of the form $y=k$ where $k$ the y-coordinate of all the points on the line. The line has a slope of zero therefore the line must be horizontal. The horizontal line passes through the point $(5, 1)$. The y-coordinate of the point $(5, 1)$ is $1$. Thus, the equation of the line is $y=1$.