9702/F/M/22/12/Q.28/A level physics past paper question/diffraction grating/superposition of waves

Описание: A parallel beam of light consists of light of wavelength 420 nm and light of wavelength 630 nm.
The light is incident normally on a diffraction grating.
The diffraction maxima for the two wavelengths overlap only at an angle of 31° from the direction of the incident light beam.
What could be the line spacing of the diffraction grating?