def get_z(coeffs, x, y):
    """
    Calculate and return the height z given the coefficients and ordinates of ax + by + c = z

    :param coeffs:  numpy ndarray of coefficients a, b, c, in the order (c, a, b)
    :param x:       x value
    :param y:       y vakye

    :return:    the calculated z value
    """
    assert isinstance(coeffs, tuple) and 3 == len(coeffs)
    assert all([isinstance(v, float) for v in coeffs])
    assert isinstance(x, float)
    assert isinstance(y, float)

    a, b, c = coeffs
    z = a*x + b*y + c
    # print z

    return z