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The points (-2,9) and (2,r) lie on a line with slope -3. Find the missing coordinate r

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The formula of a slope:


m=(y_2-y_1)/(x_2-x_1)

We have the points (-2, 9) and (2, r) and the slope m = -3. Substitute:


(r-9)/(2-(-2))=-3\\\\(r-9)/(2+2)=-3\\\\(r-9)/(4)=-3\qquad\text{multiply both sides by 4}\\\\r-9=-12\qquad\text{add 9 to both sides}\\\\\boxed{r=-3}

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