def _Backward3a_T_Ps(P, s):
    """Backward equation for region 3a, T=f(P,s)

    Parameters
    ----------
    P : float
        Pressure, [MPa]
    s : float
        Specific entropy, [kJ/kgK]

    Returns
    -------
    T : float
        Temperature, [K]

    References
    ----------
    IAPWS, Revised Supplementary Release on Backward Equations for the
    Functions T(p,h), v(p,h) and T(p,s), v(p,s) for Region 3 of the IAPWS
    Industrial Formulation 1997 for the Thermodynamic Properties of Water and
    Steam, http://www.iapws.org/relguide/Supp-Tv%28ph,ps%293-2014.pdf, Eq 6

    Examples
    --------
    >>> _Backward3a_T_Ps(20,3.8)
    628.2959869
    >>> _Backward3a_T_Ps(100,4)
    705.6880237
    """
    I = [-12, -12, -10, -10, -10, -10, -8, -8, -8, -8, -6, -6, -6, -5, -5, 
        -5, -4, -4, -4, -2, -2, -1, -1, 0, 0, 0, 1, 2, 2, 3, 8, 8, 10]
    J = [28, 32, 4, 10, 12, 14, 5, 7, 8, 28, 2, 6, 32, 0, 14, 32, 6, 10, 36,
        1, 4, 1, 6, 0, 1, 4, 0, 0, 3, 2, 0, 1, 2]
    n = [1500420082.63875, -159397258480.424, 0.000502181140217975, -
        67.2057767855466, 1450.58545404456, -8238.8953488889, -
        0.154852214233853, 11.2305046746695, -29.7000213482822, 
        43856513263.5495, 0.00137837838635464, -2.97478527157462, 
        9717779473494.13, -5.71527767052398e-05, 28830.794977842, -
        74442828926270.3, 12.8017324848921, -368.275545889071, 
        6647689047791770.0, 0.044935925195888, -4.22897836099655, -
        0.240614376434179, -4.74341365254924, 0.72409399912611, 
        0.923874349695897, 3.99043655281015, 0.0384066651868009, -
        0.00359344365571848, -0.735196448821653, 0.188367048396131, 
        0.000141064266818704, -0.00257418501496337, 0.00123220024851555]
    Pr = P / 100
    sigma = s / 4.4
    suma = 0
    for i, j, ni in zip(I, J, n):
        suma += ni * (Pr + 0.24) ** i * (sigma - 0.703) ** j
    return 760 * suma
