Solving a problem in physics section 34 point 99 optics

99. In the diffraction grating having 200 lines per 1 mm, the surface normal to its incident monochromatic light of wavelength λ = 500 nm. Near lattice converging lens placed in the focal plane of which is a screen on which is projected a diffraction pattern. Determine the distance L from the lens of the screen, if the first main peak is observed at a distance b = 10 cm from the center.