Because the number of digits in 100 is 3 larger than the single digit, the number on each digit is x-3.
The original numerical expression is 100x+10 y+x-3.
Interchange between hundredth and single digit: hundredth x-3, unit x.
The exchanged numbers are expressed as:100 (x-3)+10y+x.
Subtract a decimal from a large number:100x+10y+x-3-[100 (x-3)+10y+x] = 297.
The difference is a fixed number 297, which has nothing to do with x y.
297+ 792= 1089