Which statement is true about the graphs of the two lines y = –6 and x = ?
a The lines are perpendicular to each other because the graph of y = –6 is a horizontal line with a slope that is undefined, and the graph of x = is a vertical line with a slope of 0.
b The lines are perpendicular to each other because the graph of y = –6 is a vertical line with a slope that is undefined, and the graph of x = is a horizontal line with a slope of 0.
c The lines are perpendicular to each other because the graph of y = –6 is a vertical line with a slope of 0, and the graph of x = is a horizontal line with a slope that is undefined.
d The lines are perpendicular to each other because the graph of y = –6 is a horizontal line with a slope of 0, and the graph of x = is a vertical line with a slope that is undefined.

I would go with D.

Try drawing it out. When you do you realize that y=6 is a horizontal line and x=? would have to be a vertical line because a line that it x= anything is a vertical. Because we are not given the "?" the slope is undefined.

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