Step-by-step explanation:
If you know the frequency of the photon, you can calculate the wavelength using the equation λ=cν where c is the speed of light and ν is the frequency.
λ=cν
f=T^-1,f=1/7.81×1014s
c=3×10^8m/s
λ=3×10^8m/s×1/7.81.×1014s
λ=37881.94m