What is the slope of a line?

The slope of a line refers to how steep the line is. Given two points, (x1,y1) and (x2,y2), the slope can be found by this formula: (y2-y1) / (x2-x1). If the number is positive, the line goes up. If the slope is negative, the line goes down. If the slope is zero, the line is horizontal and if the slope is undefined because the denominator is 0, the line is vertical.