The formula for calculating the slope of a line when given two points, (x1, y1) and (x2, y2), is the difference in y coordinates divided by the difference in x coordinates. The slope, denoted as the letter m, equals (y2 - y1)/(x2 - x1).
This formula is equally valid in the form, m = (y1 - y2)/(x1 - x2).
Slope-intercept form, y = mx + b, and point-slope form, (y - y1) = m(x - x1), both represent linear equations used to solve and then graph lines.
Parallel lines are those with the same slope, and perpendicular lines have slopes that are the negative reciprocal of each other.