Chapter 8 - Coordinate Geometry and Linear Systems - 8.3 - Slope of a Line - Problem Set 8.3 - Page 349: 20

$m=-\dfrac{b}{a}$

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