We can solve this together! We're beginning with 43899 and removing 20666 from it.

Step 1: We'll start by subtracting the digit 6 and the borrow 0 from 9 in column 1 and get 3.
3 is the first digit of our result.

Step 2: We'll start by subtracting the digit 6 and the borrow 0 from 9 in column 2 and get 3.
3 is the next digit of our result.

Step 3: We'll start by subtracting the digit 6 and the borrow 0 from 8 in column 3 and get 2.
2 is the next digit of our result.

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

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

So, 43899 - 20666 = 23233.