No problem, let's work through this together. We're starting with 409091 and are subtracting 9920 from it.

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

Step 2: We'll start by subtracting the digit 2 and the borrow 0 from 9 in column 2 and get 7.
7 is the next digit of our result.

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

Step 4: We'll start by subtracting the digit 9 and the borrow 1 from 9 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 0 in column 5 and get -1.
We add -1 to 10 and get 9 as the next digit of the result.

Step 6: We'll start by subtracting the digit 0 and the borrow 1 from 4 in column 6 and get 3.
3 is the next digit of our result.

So, 409091 - 9920 = 399171.