Alright, ready to do some subtraction? We're taking 878396 and subtracting 8455 from it.

Step 1: We'll start by subtracting the digit 5 and the borrow 0 from 6 in column 1 and get 1.
1 is the first digit of our result.

Step 2: We'll start by subtracting the digit 5 and the borrow 0 from 9 in column 2 and get 4.
4 is the next digit of our result.

Step 3: We'll start by subtracting the digit 4 and the borrow 0 from 3 in column 3 and get -1.
We add -1 to 10 and get 9 as the next digit of the result.

Step 4: We'll start by subtracting the digit 8 and the borrow 1 from 8 in column 4 and get -1.
We add -1 to 10 and get 9 as the next digit of the result.

Step 5: We'll start by subtracting the digit 0 and the borrow 1 from 7 in column 5 and get 6.
6 is the next digit of our result.

Step 6: We'll start by subtracting the digit 0 and the borrow 0 from 8 in column 6 and get 8.
8 is the next digit of our result.

So, 878396 - 8455 = 869941.