diffraction problem

A student holds a laser that emits light of wavelength 633nm. The beam passes through a pair of slits separated by 0.300mm, in a glass plate attached to the front of the laser. The beam then falls perpendicularly on a screen, creating an interference pattern on it. The student begins to walk directly toward the screen at 3.00m/s. The central maximum on the screen is stationary. Find the speed of the first-order maxima on the screen.