def _Backward3a_T_Ph(P, h):
    """Backward equation for region 3a, T=f(P,h)

    Parameters
    ----------
    P : float
        Pressure [MPa]
    h : float
        Specific enthalpy [kJ/kg]

    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 2

    Examples
    --------
    >>> _Backward3a_T_Ph(20,1700)
    629.3083892
    >>> _Backward3a_T_Ph(100,2100)
    733.6163014
    """
    I = [-12, -12, -12, -12, -12, -12, -12, -12, -10, -10, -10, -8, -8, -8, -8,
         -5, -3, -2, -2, -2, -1, -1, 0, 0, 1, 3, 3, 4, 4, 10, 12]
    J = [0, 1, 2, 6, 14, 16, 20, 22, 1, 5, 12, 0, 2, 4, 10, 2, 0, 1, 3, 4, 0,
         2, 0, 1, 1, 0, 1, 0, 3, 4, 5]
    n = [-0.133645667811215e-6, 0.455912656802978e-5, -0.146294640700979e-4,
         0.639341312970080e-2, 0.372783927268847e3, -0.718654377460447e4,
         0.573494752103400e6, -0.267569329111439e7, -0.334066283302614e-4,
         -0.245479214069597e-1, 0.478087847764996e2, 0.764664131818904e-5,
         0.128350627676972e-2, 0.171219081377331e-1, -0.851007304583213e1,
         -0.136513461629781e-1, -0.384460997596657e-5, 0.337423807911655e-2,
         -0.551624873066791, 0.729202277107470, -0.992522757376041e-2,
         -.119308831407288, .793929190615421, .454270731799386,
         .20999859125991, -0.642109823904738e-2, -0.235155868604540e-1,
         0.252233108341612e-2, -0.764885133368119e-2, 0.136176427574291e-1,
         -0.133027883575669e-1]

    Pr = P/100.
    nu = h/2300.
    suma = 0
    for i, j, n in zip(I, J, n):
        suma += n*(Pr+0.240)**i*(nu-0.615)**j
    return 760*suma