Okay, let's tackle this math problem. We're starting with 89840 and subtracting 6399.

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

Step 2: We'll start by subtracting the digit 9 and the borrow 1 from 4 in column 2 and get -6.
We add -6 to 10 and get 4 as the next digit of the result.

Step 3: We'll start by subtracting the digit 3 and the borrow 1 from 8 in column 3 and get 4.
4 is the next digit of our result.

Step 4: We'll start by subtracting the digit 6 and the borrow 0 from 9 in column 4 and get 3.
3 is the next digit of our result.

Step 5: We'll start by subtracting the digit 0 and the borrow 0 from 8 in column 5 and get 8.
8 is the next digit of our result.

So, 89840 - 6399 = 83441.