Intermediate Algebra (12th Edition)

$m=\text{undefined}$
Using $m=\dfrac{y_1-y_2}{x_1-x_2}$ or the Slope Formula, the slope of the line through $( -1,5 )$ and $( -1,-4 )$ is \begin{array}{l}\require{cancel} m=\dfrac{5-(-4)}{-1-(-1)} \\\\ m=\dfrac{5+4}{-1+1} \\\\ m=\dfrac{9}{0} \\\\ m=\text{undefined} .\end{array}