Use $h=6.626\times 10^{-34}\mathrm{J}\mathrm{s}$, $c=3.0\times 10^8\mathrm{m}{\mathrm{s}}^{-1}$ and $1\mathrm{eV}=1.6\times 10^{-19}\mathrm{J}$. A hydrogen atom is in the $n=2$ level. What is the longest wavelength of light, in nm (one decimal), that can just ionise it?

Numerical answer type, answer in nm, tolerance 0.3.