Let's dive into this subtraction. We'll start with 217024 and subtract 81021 from it.

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

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

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

Step 4: We'll start by subtracting the digit 1 and the borrow 0 from 7 in column 4 and get 6.
6 is the next digit of our result.

Step 5: We'll start by subtracting the digit 8 and the borrow 0 from 1 in column 5 and get -7.
We add -7 to 10 and get 3 as the next digit of the result.

Step 6: We'll start by subtracting the digit 0 and the borrow 1 from 2 in column 6 and get 1.
1 is the next digit of our result.

So, 217024 - 81021 = 136003.