Scientists shine a laser beam on a 35-m@wide slit and produce a diffraction pattern on a screen

Scientists shine a laser beam on a 35-μm@wide slit and produce a diffraction pattern on a screen 70 cm behind the slit. Careful measurements show that the intensity first falls to 25% of maximum at a distance of 7.2 mm from the center of the diffraction pattern. What is the wavelength of the laser light?