Answer:
undefined
Explanation:
You want the slope of the line through points (-9 ,5) and (-9, -6).
Slope
The slope is the ratio of the "rise" to the "run" between points on the line. It is given by the formula ...
m = (y2 -y1)/(x2 -x1)
Application
For the given points, the slope is ...
m = (-6 -5)/(-9 -(-9)) = -11/0 = undefined
The ratio involves division by zero, so we say the slope is undefined.
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Additional comment
Any vertical line will have undefined slope. (The slope-intercept or point-slope forms of the equation of a line cannot be used to describe it.)