Answer
91.2 nm
Work Step by Step
The longest wavelength photon has an energy equal to the ionization energy of 13.6 eV.
$$ E_{ionization} =hf = \frac{hc}{\lambda}$$
In this problem, the value of hc is useful. Use the result stated in Problem 29, $hc=1240\;eV \cdot nm$.
$$ \lambda=\frac{hc}{E_{ionization}}=\frac{1240\;eV \cdot nm }{13.6eV}=91.2 nm$$