Angel, the short answer is that the sum of numbers that you got by subtracting the two numbers will always be divisible by 9. So after leaving out the one that you circled (if you circle a 0 it will screw everything up tho) you add the remaining numbers until you get a single digit number. The difference between that number and 9 is the number you circled.
For example:
6543-3456 = 3087
If you circle the 8 then you are left with 307. 3+0+7 = 10. 1+0 = 1. 9-1 = 8, which is the number you circled. If you circled the 3 then 0+8+7 = 15, 1+5 = 6, 9-6 = 3. If you circled the 7 then 3+0+8 = 11, 1+1 = 2, 9-2 = 7.
Jumbling the numbers is irrelevant since you are adding them.
It really is pretty simple as long as you know what the trick is