What is the slope of the line that contains the points (5, –9) and (5, 9)?
a. 0
b.
c.
d. the slope is undefined.

Slope(m)=(y2-y1)/(x2-x1)
here
(5,-9)=(x1,y1)
(5,9)=(x2,y2)
so, m= (9+9)/(5-5)=infinity
it means the line is perpendicular with the x-axis
because tan90 is equal to infinity
so in order to make slope infinity the line should make 90 angle with positive x axis