(a) Calculate the longitudinal mode wavelength separation () in the case of semiconductor laser with refractive index of nr = 3.6 and a cavity length of 200 m for the emission wavelength of = 800 nm. (b) If the gain spectrum has a bandwidth of 6 nm. How many lasing mode will appear within the gain spectrum? (c) To make the (longitudinal) single-mode lasers (a single emission wavelength) from the same materials, calculate the maximum cavity length of the laser.

I need help with part c please!