What is the wavelength λ of the photon that has been released in Part B?

ΔE = −2.04×10−18 J
The energy of the photon emitted by the electron is related to its wavelength by

E=hcλ

where λ is the wavelength in meters, 6.626×10−34 J⋅s is Planck's constant, and 3.00×108 m/s is the speed of light in a vacuum.