For each of the pairs of points, we calculate the slope.

(a)(-7,8) and (-7,0)
![\begin{gathered} \text{Slope}=(8-0)/(-7-(-7))=(8)/(-7+7) \\ =(8)/(0) \\ =\text{Undefined} \end{gathered}]()
The slope of the line joining the points (-7,8) and (-7,0) is undefined.
(b)(6,-3) and (-4,-3)

The slope of the line joining the points (6,-3) and (-4,-3) is zero.
(c)(2,4) and (5,1)

The slope of the line joining the points (2,4) and (5,1) is negative.
(d)(3,5) and (-1,2)

The slope of the line joining the points (3,5) and (-1,2)) is positive.