At 7 p. M last night, the tempurture was 10. At 7 a. M the next mourning, the tempurture was - 2 by how much did the tempurture change from 7 p. M to a. M

It's asking how much the temperature fell so if it started at 10 and dropped to negative 2 this is one of the few cases you add them together and get 12 in other cases that aren't asking for a total like 10 + (-2) it would be 8 but in this case it's 10+2 since its asking for a temperature drop instead of it being an algebra problem.

10 - 2 = 9

the answer is:

the temperature decreased by 9 degrees