When you add 6 + 4, you get the last two digits, and when you subtract 6 - 4, you get the first digit, making it 210. Same with 9 + 2 = 711. You add 9 + 2 to get 11, and you subtract those numbers, you get 7.
Subtracting the addends (i.e. 6 and 4) gives you the hundreds (or the first digit) while adding the addends gives you the tens and ones (2nd and 3rd digit) So 6+4=210 is 6-4 = 2 and 6+4 = 10 hence 210
