4-2=2. This is an example of a very simple subtraction problem. We start with 4 and want to see how many we get if we take away 2 (4-2). Of course, the answer is 2.
Once you have gotten this down, time to work on borrowing. Borrowing is when a column(from the bigger number)is smaller than the other number in the same column. I.e. 12-6. You can do this problem by taking the 1 in 12 and making it ten. Then add it the ones column(2) to get twelve. Then subtract 12-6. If the larger number had more than 1 in the tens column, you would still only borrow 1 ten. It's a pretty hard and if you don't get it, try another site or ask a parent