Answer:
6 minutes
Step-by-step explanation:
We are given that an altitude a (in feet) of a plane t minutes after liftoff is given by
We have to find that how many minute after liftoff is the plane at an altitude of 21,000 feet.
Using subtraction property of equality
Substitute the value of a then we get
minutes
Hence, 6 minutes after liftoff is the plane at an altitude of 21,000 feet.