When Emily woke up, the temperature was 8 degrees F. By noon, the temperature had risen by 3 degrees F. It rose another 6 degrees in the afternoon to reach the day's high temperature. By the time Emily went to bed, the temperature had dropped 21 degrees F from the high temperature. What was the temperature when Emily went to bed?

The temperature when Emily went to bed was -4°F.
Step-by-step explanation:
8°F + 3°F = 11°F (noon)
The temperature had risen by 3°F by noon. This means to add 3.
noon + 6°F = the day's high temperature
11°F + 6°F = 17°F (the day's high temperature)
The temperature rose another 6°F in the afternoon. This means add 6 to the temperature from noon.
high temperature - 21°F = temperature at bed
17°F - 21°F = -4°F
The temperature dropped by 21°F when Emily went to bed. This means to subtract 21 from the day's high temperature.
Risen, rose, and increase mean addition.
Decrease, dropped, and decline mean subtraction.
Hope this helps :)