Complex sentence: At the end of the 75th lap Lagorce was ordered by the team to bring the remaining CLR directly to its garage; upon the car's arrival AMG-Mercedes shut the last of their three garage doors signifying their official retirement from the event.
Split into simpler sentences: At the end of the following race lap Franck Lagorce was ordered by the AMG-Mercedes team to bring the remaining CLR to its garage. Upon the parking of the third CLR, AMG-Mercedes shut the last of their three garage doors signifying their official withdrawal from the event.