Solution to task 513 from Chertov´s for extramural stud

Solution to task 513 from Chertov´s for extramural stud

513. A normally parallel beam of white light is incident on the diffraction grating. Spectra of the third and fourth orders are partially overlapped. At what wavelength in the fourth-order spectrum is the boundary (λ = 780 nm) of the third-order spectrum superimposed?