A linear equation is one in which the variables are raised to the first power. The graph of a linear equation is a line. Slope a measurement of the steepness of a line which is calculated by comparing its vertical change (rise) to its horizontal change (run) Slope of a line, m = rise run m y. , x1 x2 (x1,y1) (x2, y2) rise = y2 - y1 run = x2 - x1. If x1 = x2, then the line is vertical and the slope is undefined. Equation of a vertical line: if a line goes through point (a, y) and is vertical, then the equation is given by x = a . Equation of a horizontal line: if a line goes through point (x, b) and is horizontal, then the equation is given by y = b. slope is zero. Point-slope form of the equation of a line: y - y1 = m(x - x1)