An airplane is flying at 38,800 feet above sea level. The airplane starts to descend at a rate of 18

An airplane is flying at 38,800 feet above sea level. The airplane starts to descend at a rate of 1800 feet per minute. Let m be the number of minutes. Which of the following expressions describe the height of the airplane after any given number of minutes?

Let m be the number of minutes. Since a descent equals a drop in altitude, we subtract this in our Altitude function A(m):
A(m) = 38,800 - 1800m