y = -2x + 1
y = -2x + 2
y = -2x + 3
these are all parallel lines because they have the same slopes (-2) but different y intercepts.
y = 3x + 4
y = -1/3x + 3
these are perpendicular lines because they have negative reciprocal slopes.
slope of a line parallel to the x axis : If it is parallel to the x axis, it is a horizontal line and has a slope of 0.
slope of a line perpendicular to the x axis : if it is perpendicular to the x axis, it is a vertical line with a slope that is undefined.
slope of line parallel to the y axis : vertical line, undefined slope
slope of a line perpendicular to the y axis : horizontal line, slope of 0