Please explain these question in simple detailed steps also in simple steps of rearranging the formulas if needed.

2) In a double-slit experiment, the slit separation is 2.0 mm, and two wavelengths, 750 nm and 900 nm, illuminate the slits. A screen is placed 2.0 m from the slits. At what distance from the central maximum on the screen will a bright fringe from one pattern first coincide with a bright fringe from the other? Answer: 4.5 mm 3) The distance between the ruled lines on a diffraction grating is 1770 nm. The grating is illuminated at normal incidence with a parallel beam of white light in the 400 nm to 700 nm wavelength band. What is the longest wavelength that appears in the third order spectrum? Answer: 590 nm