Chapter 8 - Coordinate Geometry and Linear Systems - 8.3 - Slope of a Line - Problem Set 8.3 - Page 349: 16

$m=\text{undefined}$

Using $m=\dfrac{y_1-y_2}{x_1-x2}$ the slope of the line determined by the given points, $( -4,-5 ) \text{ and } ( -4,9 ),$ is \begin{array}{l}\require{cancel} m=\dfrac{-5-9}{-4-(-4)} \\\\ m=\dfrac{-5-9}{-4+4} \\\\ m=\dfrac{-14}{0} \\\\ m=\text{undefined} .\end{array}