Notice that from 7PM to 10 PM, there are 3 hours difference.
At 7 PM the temperature was -5° F, then one hour later, (at 8PM it was -2.5°F less, that is: -7.5°F.
One hour later (9PM) : -7.5°F - 2.5°F = -10°F
Another hour later (10PM) : -10°F - 2.5°F = -12.5°F
The problem could also be solved in one step by multiplying the rate of change times "3" (due to the 3 hours elapsed between 7PM and 10PM), and using it to modify the starting value (-5°F):
