how do you do parallel and perpendicular lines

If you are talking about graphing equations,

parallel lines have the same slope (for example y = 2x + 9 is parallel to y = 2x - 7 because both have the same slope, 2)

and perpendiculr lines have slopes that are negative reciprocals (for example y = 2x + 9 is perpendicular to y = (-1/2)x - 7, because the slopes are negative reciprocals).

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