Kadeesha spends a winter day recording the temperature once every three hours for
science class. At 9 am, the temperature was -6.9°F. Between 9am and noon, the
temperature increased by 5.4°F. Between noon and 3pm, the temperature increased
by 7.2°F. Between 3pm and 6pm, the temperature dropped 15.5°F. What was the
temperature at 6pm?