def primary_function(x1, y1, x2, y2):
    """
    a = (y2- y1) / (x2 -x1)
    b = y1 - ax1
    Return
    y = ax + b
    ----------
    a: float
    b: float
    """
    a = (y2 -y1) / ((x2 -x1))
    b = y1 - a * x1

    return [a, b]