[tex]m = \frac{y_2 - y_1}{x_2 - x_1} [/tex]
Substitute the coordinate points value in the formula.
[tex]m = \frac{3 - ( - 12)}{ - 9 - 6} \\ m = \frac{3 + 12}{ - 15} \\ m = \frac{15}{ - 15} \\ m = - 1[/tex]
From the slope-intercept form y = mx+b where m = slope and b = y-intercept. Therefore slope = m = -1
[tex] \large \boxed{m = - 1}[/tex]