Answer:
-21°C
Explanation:
Since the plane is flying at an altitude of 5 kilometers, then we have to multiply 6.4 by each kilometer that the plane's altitude increased (5). After that, we will end up with 32°C that we have to subtract from the ground temperature of 11°C. Once we do that, we get -21°C which means that the temperature at an altitude of 5 kilometers is -21°C.
(sorry if this is confusing but I hope this helped)