To find the balloon's change in elevation after 3 minutes if it descends 800 feet per minute, you can simply multiply the descent rate by the number of minutes:
Change in elevation = Descent rate × Time
Change in elevation = 800 feet/minute × 3 minutes
Change in elevation = 2400 feet
So, the balloon's change in elevation after 3 minutes of descending at a rate of 800 feet per minute is 2400 feet.