Step-by-step explanation:
starting point :
9 am -8.4°F
from 9 am to 12pm the temperature increased by 2.6°F.
12 pm -8.4 + 2.6 = -5.8°F
from 12pm to 3pm the temperature increased by 1.2°F.
3 pm -5.8 + 1.2 = -4.6°F
from 3pm to 6pm the temperature decreases by 9.9°F.
6 pm -4.6 - 9.9 = -14.5°F
we could have done the short approach.
the temperature changed overall by
2.6 + 1.2 - 9.9 = 3.8 - 9.9 = -6.1°F
and so, the final temperature was
-8.4 + -6.1 = -8.4 - 6.1 = -14.5°F