What is the slope of the graph of the line x = –1?
Pick two points on the line, like (-1,0) and (-1, 5)
m = (y2 - y1)/(x2 - x1)
m= (5 - 0) / (-1 - -1)
5/0 is undefined, because you can't divide by zero. So the slope of the line is undefined, and the line is vertical.
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