Hi I have problem in below mention question can any body help me how I can solve

Ships arrive at a port at a rate of one in every 3 hours, with a negative exponential distribution of inter arrival times. The time a ship occupies a berth for unloading and loading has a negative exponential distribution with an average of 12 hours. If the average delay of ships waiting for berths is to be kept below 6 hours, how many berths should there be at the port