Question

The points (6,-6) and (9, r) lie on a line w slope -1. What is the missing coordinate r?

Answer 1

r=-9.

you can use the formula for slope to find the answer.
$slope = \frac{y_2-y_1}{x_2-x_1} \\ -1= \frac{r--6}{9-6} = \frac{r+6}{3} \\ -1=\frac{r+6}{3} \\ -3=r+6 \\ r=-9$