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 = [-1.33645667811215e-07, 4.55912656802978e-06, -1.46294640700979e-05,
        0.0063934131297008, 372.783927268847, -7186.54377460447, 
        573494.7521034, -2675693.29111439, -3.34066283302614e-05, -
        0.0245479214069597, 47.8087847764996, 7.64664131818904e-06, 
        0.00128350627676972, 0.0171219081377331, -8.51007304583213, -
        0.0136513461629781, -3.84460997596657e-06, 0.00337423807911655, -
        0.551624873066791, 0.72920227710747, -0.00992522757376041, -
        0.119308831407288, 0.793929190615421, 0.454270731799386, 
        0.20999859125991, -0.00642109823904738, -0.023515586860454, 
        0.00252233108341612, -0.00764885133368119, 0.0136176427574291, -
        0.0133027883575669]
    Pr = P / 100.0
    nu = h / 2300.0
    suma = 0
    for i, j, n in zip(I, J, n):
        suma += n * (Pr + 0.24) ** i * (nu - 0.615) ** j
    return 760 * suma
