Answer
$2.6\times10^{-12}m$
Work Step by Step
Find the wavelength from the energy difference between the states.
$$\Delta E = hf=\frac{hc}{\lambda}$$
$$\lambda=\frac{hc}{\Delta E}$$
$$ =\frac{(6.63\times10^{-34}J \cdot s)(3.00 \times10^8 m/s)}{(0.48 MeV)(1.60\times10^{-13}J/MeV)}$$
$$=2.6\times10^{-12}m$$
