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 = [-0.182575361923032e-1, -0.125229548799536, 0.592290437320145,
         0.604769706185122e1, 0.238624965444474e3, -0.298639090222922e3,
         0.512250813040750e-1, -0.437266515606486, 0.413336902999504,
         -0.516468254574773e1, -0.557014838445711e1, 0.128555037824478e2,
         0.114144108953290e2, -0.119504225652714e3, -0.284777985961560e4,
         0.431757846408006e4, 0.112894040802650e1, 0.197409186206319e4,
         0.151612444706087e4, 0.141324451421235e-1, 0.585501282219601,
         -0.297258075863012e1, 0.594567314847319e1, -0.623656565798905e4,
         0.965986235133332e4, 0.681500934948134e1, -0.633207286824489e4,
         -0.558919224465760e1, 0.400645798472063e-1]

    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