A photon of 300 nm is absorbed by a gas which then re – emits two photons. One re- emitted photon has wavelength of 496 nm. Wavelength of other re – emitted photon is 

$\lambda$of photon absorbed = 300 nm $\lambda$ of I photon re – emitted out = 496 nm

Let  $\lambda$ of II photon re – emitted out ${\lambda }_H$

$\because$Total energy absorbed = Total energy re – emitted out

$\therefore \frac{hc}{300\times {10}^{-9}}=\frac{hc}{496\times {10}^{-9}}+\frac{hc}{{\lambda }_H}$

$\therefore {\lambda }_H=759\times {10}^{-9,}m=759nm$