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)

m= 5/0

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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