The rate of change is the slope, which is calculated by taking two point on the line and dividing the difference in the horizontal coordinate into the difference in the vertical coordinate.
So if the points are (x1, y1) and (x2, y2) the slope is (y2-y1)/(x2-x1). The slope can be positive or negative, which is the “lean” of the line to the right (positive) or to the left (negative).