def slope_formula(x1, x2, y1, y2):
    """
    definition: func(x1, x2, y1, y2) {...}
    objective : returns the slope (m) from the slope formula
                m = y2 - y1 / x2 - x1
    """
    slope = (y2 - y1) / (x2 - x1)
    return slope
