The way to do this mathematically is by incorporating the Order of Operations.(Please Excuse My Dear Aunt Sally) This stands for (do first:) Parenthesis, then exponents, after that multiplication, now division, then addition, and finally subtraction. The side on the right then equals -5 while the side on the right equals -6. The greater value is the left side. If you have any other questions, just ask me!
Answer:

Step-by-step explanation:
1/3 + 1/6
---Find a common denominator, or the least common multiple of the given denominators. In this case, that would be 6.
1/3 = 2/6
1/6 = 1/6
---Now add the fractions
2/6 + 1/6
3/6
---Simplify/Reduce the fraction
3/6
1/2
Answer: 1/2
1/3 + 1/6 = 2/6 + 1/6 = 3/6 (or 1/2)
Hope this helps!
-2° was the initial temperature as it dropped to -5°
-2-5=-7
Hope this helped