Average speed = total distance/total time so you need to find the ship's total distance traveled and the total time the ship traveled.
To find the total distance traveled, first find the distance for the return trip.
Distance = (rate)(time) so if the trip back took 5 hours and the ship averaged 30 mph, then the distance the ship traveled on the trip back was (30 mph)(5 h) = 150 miles.
The distances for both trips are the same so the distance there was also 150 miles.
Since the ship traveled a total distance of 150 miles + 150 miles = 300 miles and the total time is 6 hours + 5 hours = 11 hours, then the ship's average speed was: