def lin_f(xy_s, xy_e):
    """Find a function representing for a line:
                    y = ax + b

    :param: xy_s: tuple
        start point coordinate of the line
    :param: xy_s: tuple
        end point coordinate of the line

    """
    a = (xy_e[1] - xy_s[1]) / (xy_e[0] - xy_s[0])
    b = xy_e[1] - a * xy_e[0]

    return a, b