You talk to a friend who lives in a different state. she tells you that today it is 12° below zero where she lives. you tell her it is 54° where you live. what is the difference in temperatures?

12 degrees below zero tells you we have a negative number, -12. The word "Difference" means we are subtracting. So your equation will be 54 - (-12). Two minuses/negatives back-to-back turn into a plus so the equation is now 54 + 12 = 66.

There is a 66 degree difference in temperatures assuming the temperatures given are using the same units (I mean one is not Celsius and the other Fahrenheit). 

Friend = -12°C
you = -54°C

difference = 54 - 12
= 42 degrees.