By convention, a temperature rise is additive and a temperature drop is subtractive.
The initial temperature on Monday was -8 °C.
It rises 3 °C, thus the new temperature was -8 °C + 3 °C = -5 °C.
Finally, the temperature drops by 9 °C, thus the final temperature was
-5 °C - 9 °C = -14 °C.
Answer:
The temperature was -14 °C when you woke up on Tuesday