We can conclude the following information:
Every 3 hours the temperature changes.
At 9 AM, the temperature was -1.6°F:
9 AM = -1.6°F =>
Between 9 AM and noon, the temperature rose 6.6°F:
12 PM = -1.6 + 6.6 = 5°F =>
Between noon and 3 PM, the temperature increased by 2.8°F:
3 PM = 5 + 2.8 = 7.8°F =>
Between 3 PM and 6 PM, the temperature went down 10.5°F:
6 PM = 7.8 - 10.5 = -2.7°F
What was the temperature at 6 PM?
-2.7°F