Well... You didn't specify whether or not it is Farenheight or Celsius, So im going to show both.
Farenheight:
-5 + 12 = 7
Because
... A simple breaking way to show it would be: 12-5=7 -5+5=0 0+7 = 7
Celsius:
Its the same :)
If you were going to convert it to Farenheight though:
-5 Celsius = 23* Farenheight.
12 Celsius = 53.6* Farenheight
23
+56.3
=-------
79.3
Hope this helped and I don't sound to nerdy