The term "difference" os the result of subtracting one number from another.
So in other terms you want to know what would be the result of subtracting -4.6 from 13.5 :
13.5 - -4.6 = Difference between the two temperatures
Two minus signs turn into a positive sign so :
13.5 - -4.6 =
13.5 + 4.6 =
18.1 = Difference between the two temperatures
Your answer is : The difference between the two temperatures is 18.1