def _Backward2a_T_Ph(P, h):
    """Backward equation for region 2a, T=f(P,h)

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

    Returns
    -------
    T : float
        Temperature [K]

    References
    ----------
    IAPWS, Revised Release on the IAPWS Industrial Formulation 1997 for the
    Thermodynamic Properties of Water and Steam August 2007,
    http://www.iapws.org/relguide/IF97-Rev.html, Eq 22

    Examples
    --------
    >>> _Backward2a_T_Ph(0.001,3000)
    534.433241
    >>> _Backward2a_T_Ph(3,4000)
    1010.77577
    """
    I = [0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 2, 2,
         3, 3, 4, 4, 4, 5, 5, 5, 6, 6, 7]
    J = [0, 1, 2, 3, 7, 20, 0, 1, 2, 3, 7, 9, 11, 18, 44, 0, 2, 7, 36, 38, 40,
         42, 44, 24, 44, 12, 32, 44, 32, 36, 42, 34, 44, 28]
    n = [0.10898952318288e4, 0.84951654495535e3, -0.10781748091826e3,
         0.33153654801263e2, -0.74232016790248e1, 0.11765048724356e2,
         0.18445749355790e1, -0.41792700549624e1, 0.62478196935812e1,
         -0.17344563108114e2, -0.20058176862096e3, 0.27196065473796e3,
         -0.45511318285818e3, 0.30919688604755e4, 0.25226640357872e6,
         -0.61707422868339e-2, -0.31078046629583, 0.11670873077107e2,
         0.12812798404046e9, -0.98554909623276e9, 0.28224546973002e10,
         -0.35948971410703e10, 0.17227349913197e10, -0.13551334240775e5,
         0.12848734664650e8, 0.13865724283226e1, 0.23598832556514e6,
         -0.13105236545054e8, 0.73999835474766e4, -0.55196697030060e6,
         0.37154085996233e7, 0.19127729239660e5, -0.41535164835634e6,
         -0.62459855192507e2]

    Pr = P/1
    nu = h/2000
    T = 0
    for i, j, ni in zip(I, J, n):
        T += ni * Pr**i * (nu-2.1)**j
    return T