The formula of a slope:
[tex]m=\dfrac{y_2-y_1}{x_2-x_1}[/tex]
We have the points (-2, 9) and (2, r) and the slope m = -3. Substitute:
[tex]\dfrac{r-9}{2-(-2)}=-3\\\\\dfrac{r-9}{2+2}=-3\\\\\dfrac{r-9}{4}=-3\qquad\text{multiply both sides by 4}\\\\r-9=-12\qquad\text{add 9 to both sides}\\\\\boxed{r=-3}[/tex]
