What is the slope of the line perpendicular to x = -3?

What is the slope of a line that is perpendicular to the line x = –3?

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The graph of x = -3 is a vertical line passing through -3 on the x-axis.  
All vertical lines have slopes that are undefined.

A line perpendicular to a vertical line is a horizontal line.  Again, vertical and horizontal lines are perpendicular.
All horizontal lines have slopes that are 0.

The slope of the line that is perpendicular to the vertical line x = -3 would be 0.

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