The formula of a slope:
[tex]m=\dfrac{y_2-y_1}{x_2-x_1}[/tex]
We have the points (6, 2) and (9, r), the slope m = -1. Substitute:
[tex]-1=\dfrac{r-2}{9-6}\\\\-1=\dfrac{r-2}{3}\qquad\text{multiply both sides by 3}\\\\-3=r-2\qquad\text{add 2 to both sides}\\\\\boxed{r=-1}[/tex]
