In this case, the airplane is flying at 35000 feet and descend 2500 feet every minute. That means the airplane descend rate is 2500 feet/min. Since the duration is m minute then you need to multiply it by the descending rate. Then the formula that can count the airplane height would be:
Airplane height: 35000feet - (2500feet/min x m min) 35000feet - 2500m feet
