def slope( x1, y1, x2, y2) :
    # the slope of a line can be gotten by examining two points on its
    # line.
    #
    # the slope is the ratio of the rise to the run, where the rise
    # is the difference in y values and the run is the difference in
    # x values

    # to get the rise, I need the difference in the y values
    rise = y2 - y1

    # to get the run, I need the difference in the x values
    run = x2 - x1

    # to get the result, I need the ratio of the rise to the run
    result = rise / run
    

    # hand back the result 
    return result