Algebra
posted by CC .
An operation is commutative if you can change the order of the numbers involved without changing the result. Addition and multiplication are both commutative. Subtraction is not commutative: 2  1 is not equal to 1  2. So my question is whatâ€™s the difference between 2  1 and 1  2?

21=1 and 12=1

21 = 1 and 12 = 1.
So the difference is 2.
If you asking why it is that way, it just IS. It can be deduced logically from how subtraction is defined. 
There is no difference between 12 and 21 if you disregard the + or  in the final answer.
Sometimes when I do calculations on the calculator and I forget which number is the larger, I will end up with a negative answer. I just remember that the answer should be positive. Look at the equation and you can see if the answer should be + or .