Solving a problem in physics section 2 point 54 optics

54. diffraction grating comprising n = 500 strokes per 1 mm is incident in the direction normal to the surface of white light. The spectrum is projected placed near the grating lens on the screen. Determine the width b of the spectrum of the first order on the screen, if the distance L is equal to the lens to the screen 3m. The boundaries of the visible spectrum λkr = 780 nm, 400 nm = λf.