the slopes of 2 lines that are parallel will be the same. the slopes of 2 lines that are perpendicular will have negative reciprocal slopes.
parallel to a line with slope 2 is 2 (because parallel lines have the same slope). perpendicular to a line with slope 2 is -1/2 (the negative reciprocal...to find the negative reciprocal, just flip the slope of the original equation and change the sign)