def _Backward2a_P_hs(h, s):
    """Backward equation for region 2a, P=f(h,s)

    Parameters
    ----------
    h : float
        Specific enthalpy, [kJ/kg]
    s : float
        Specific entropy, [kJ/kgK]

    Returns
    -------
    P : float
        Pressure, [MPa]

    References
    ----------
    IAPWS, Revised Supplementary Release on Backward Equations for Pressure
    as a Function of Enthalpy and Entropy p(h,s) for Regions 1 and 2 of the
    IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of
    Water and Steam, http://www.iapws.org/relguide/Supp-PHS12-2014.pdf, Eq 3

    Examples
    --------
    >>> _Backward2a_P_hs(2800,6.5)
    1.371012767
    >>> _Backward2a_P_hs(2800,9.5)
    0.001879743844
    >>> _Backward2a_P_hs(4100,9.5)
    0.1024788997
    """
    I = [0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 2, 3, 3, 3, 
        3, 3, 4, 5, 5, 6, 7]
    J = [1, 3, 6, 16, 20, 22, 0, 1, 2, 3, 5, 6, 10, 16, 20, 22, 3, 16, 20, 
        0, 2, 3, 6, 16, 16, 3, 16, 3, 1]
    n = [-0.0182575361923032, -0.125229548799536, 0.592290437320145, 
        6.04769706185122, 238.624965444474, -298.639090222922, 
        0.051225081304075, -0.437266515606486, 0.413336902999504, -
        5.16468254574773, -5.57014838445711, 12.8555037824478, 
        11.414410895329, -119.504225652714, -2847.7798596156, 
        4317.57846408006, 1.1289404080265, 1974.09186206319, 
        1516.12444706087, 0.0141324451421235, 0.585501282219601, -
        2.97258075863012, 5.94567314847319, -6236.56565798905, 
        9659.86235133332, 6.81500934948134, -6332.07286824489, -
        5.5891922446576, 0.0400645798472063]
    nu = h / 4200
    sigma = s / 12
    suma = 0
    for i, j, ni in zip(I, J, n):
        suma += ni * (nu - 0.5) ** i * (sigma - 1.2) ** j
    return 4 * suma ** 4
