A plane monochromatic light wave falls normally on a long rectangular slit behind which a screen is

Question:

A plane monochromatic light wave falls normally on a long rectangular slit behind which a screen is positioned at a distance b = 60 cm. First the width of the slit was adjusted so that in the middle of the diffraction pattern the lowest minimum was observed. After widening the slit by ∆h = 0.70 mm, the next minimum was obtained in the centre of the pattern. Find the wavelength