What is the slope of the line x = 4?

x = 4 has undefined slope because it's a vertical line.

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Vertical line

A line with x = 4 can be expressed as y = mx + c , that is : y = 4m + c , so slope m = y / 4 , but with x = 4 the said line is parallel to y axis and it is at a distance from yy' axis at a distance of 4 , so a line parallel to yy' axis is a perpendicular line to xx' axis so it has no slope.

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