3 minus negative 3

I think of negative numbers as cold and positive numbers as hot so

3--3

- -3 means take away 3 "degrees" of cold.  

Well if you take away cold the room must get warmer so - - = +

3 - - 3 = 3 + 3 = 6

To subtract means to add the opposite; so subracting a negative is the same as adding a positive:

3 - -3  =  3 + 3  = 6