Solving a problem in physics section 34 point 95 optics

95. At the slit width a = 0.24 mm is normally incident monochromatic light with a wavelength λ = 600 nm. The diffraction pattern is projected onto the screen is parallel to the plane of the gap, with a lens disposed near the gap. Determine the distance from the screen to the lens when the distance b between the first diffraction minima located on either side of the central maximum, equal to 1 cm.