Chapter 8 - Coordinate Geometry and Linear Systems - 8.3 - Slope of a Line - Problem Set 8.3 - Page 349: 12

$m=0$

Using $m=\dfrac{y_1-y_2}{x_1-x_2}$, the slope of the line determined by the given points, $( -3,-6 ) \text{ and } ( 5,-6 ),$ is \begin{array}{l}\require{cancel} m=\dfrac{-6-(-6)}{-3-5} \\\\ m=\dfrac{-6+6}{-8} \\\\ m=\dfrac{0}{-8} \\\\ m=0 .\end{array}