Final answer:
The time of sunset at 91 degrees west would be 6:30 pm.
Step-by-step explanation:
Given that there is a one-hour time difference for every 15 degrees of longitude, if the stated time of sunset is 6:45 pm at 90 degrees west, then at 91 degrees west, the sunset time would be 6:30 pm.
This is because moving one degree to the west results in a 4-minute time difference, and thus moving 15 degrees to the west would result in a one-hour time difference.