def _Backward2b_P_hs(h, s):
    """Backward equation for region 2b, 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 4

    Examples
    --------
    >>> _Backward2b_P_hs(2800,6)
    4.793911442
    >>> _Backward2b_P_hs(3600,6)
    83.95519209
    >>> _Backward2b_P_hs(3600,7)
    7.527161441
    """
    I = [0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 2, 2, 2, 3, 3, 3, 3, 4, 4, 5, 5, 6,
         6, 6, 7, 7, 8, 8, 8, 8, 12, 14]
    J = [0, 1, 2, 4, 8, 0, 1, 2, 3, 5, 12, 1, 6, 18, 0, 1, 7, 12, 1, 16, 1,
         12, 1, 8, 18, 1, 16, 1, 3, 14, 18, 10, 16]
    n = [0.801496989929495e-1, -0.543862807146111, 0.337455597421283,
         0.890555451157450e1, 0.313840736431485e3, 0.797367065977789,
         -0.121616973556240e1, 0.872803386937477e1, -0.169769781757602e2,
         -0.186552827328416e3, 0.951159274344237e5, -0.189168510120494e2,
         -0.433407037194840e4, 0.543212633012715e9, 0.144793408386013,
         0.128024559637516e3, -0.672309534071268e5, 0.336972380095287e8,
         -0.586634196762720e3, -0.221403224769889e11, 0.171606668708389e4,
         -0.570817595806302e9, -0.312109693178482e4, -0.207841384633010e7,
         0.305605946157786e13, 0.322157004314333e4, 0.326810259797295e12,
         -0.144104158934487e4, 0.410694867802691e3, 0.109077066873024e12,
         -0.247964654258893e14, 0.188801906865134e10, -0.123651009018773e15]

    nu = h/4100
    sigma = s/7.9
    suma = 0
    for i, j, ni in zip(I, J, n):
        suma += ni * (nu-0.6)**i * (sigma-1.01)**j
    return 100*suma**4