Write the equation of the line passing through the given pair of points. (-9, 7) and (-9, -1)

The first thing I should do with these two points is find the slope of the line.

m = (y2 - y1)/(x2 - x1)

m = (-1 - 7)/(-9 - -9)
m = -8/0

The slope is undefined. We cannot divide by 0. This makes sense because the points (-9, 7) and (-9, -1) would be right on top of each other.  This is a vertical line that has an x-intercept at -9. 

Your equation would be x = -9.