We can solve this together! We're beginning with 539999 and removing 21645 from it.

Step 1: We'll start by subtracting the digit 5 and the borrow 0 from 9 in column 1 and get 4.
4 is the first digit of our result.

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

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

Step 4: We'll start by subtracting the digit 1 and the borrow 0 from 9 in column 4 and get 8.
8 is the next digit of our result.

Step 5: We'll start by subtracting the digit 2 and the borrow 0 from 3 in column 5 and get 1.
1 is the next digit of our result.

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

So, 539999 - 21645 = 518354.