#### Answer

(a) $m=\dfrac{3}{2}$
(b) $m=0$
(c) slope is undefined

#### Work Step by Step

RECALL:
The slope $m$ of the line that contains $(x_1, y_1)$ and $(x_2, y_2)$ is given by the formula:
$$m=\dfrac{y_2-y_1}{x_2-x_1}$$
Use the formula above to obtain:
(a) $m=\dfrac{11-5}{5-1}=\dfrac{6}{4}=\dfrac{3}{2}$
(b) $m=\dfrac{4-4}{1-(-7)}=\dfrac{0}{1+7}=\dfrac{0}{8}=0$
(c) $m=\dfrac{-4-12}{6-6}=\dfrac{-16}{0}$ (slope is undefined)