def _h_3ab(P):
    """Define the boundary between Region 3a-3b, h=f(P)

    >>> "%.6f" % _h_3ab(25)
    '2095.936454'
    """
    return 0.201464004206875e4+3.74696550136983*P-0.0219921901054187*P**2+0.875131686009950e-4*P**3