1. **State the problem:** We need to find the slope of line $r$ which is perpendicular to line $q$. Line $q$ passes through points $(-91, -80)$ and $(-90, -72)$. 2. **Find the slope of line $q$:** The slope formula for two points $(x_1, y_1)$ and $(x_2, y_2)$ is $$m = \frac{y_2 - y_1}{x_2 - x_1}.$$ 3. **Calculate the slope of $q$:** Substitute the points into the formula: $$m_q = \frac{-72 - (-80)}{-90 - (-91)} = \frac{-72 + 80}{-90 + 91} = \frac{8}{1} = 8.$$ 4. **Find the slope of line $r$:** Lines that are perpendicular have slopes that are negative reciprocals of each other. If $m_q = 8$, then $$m_r = -\frac{1}{m_q} = -\frac{1}{8}.$$ 5. **Final answer:** The slope of line $r$ is $$\boxed{-\frac{1}{8}}.$$