Let's dive into this subtraction. We'll start with 72018 and subtract 68456 from it.

Step 1: We'll start by subtracting the digit 6 and the borrow 0 from 8 in column 1 and get 2.
2 is the first digit of our result.

Step 2: We'll start by subtracting the digit 5 and the borrow 0 from 1 in column 2 and get -4.
We add -4 to 10 and get 6 as the next digit of the result.

Step 3: We'll start by subtracting the digit 4 and the borrow 1 from 0 in column 3 and get -5.
We add -5 to 10 and get 5 as the next digit of the result.

Step 4: We'll start by subtracting the digit 8 and the borrow 1 from 2 in column 4 and get -7.
We add -7 to 10 and get 3 as the next digit of the result.

Step 5: We'll start by subtracting the digit 6 and the borrow 1 from 7 in column 5 and get 0.
0 is the next digit of our result.

So, 72018 - 68456 = 3562.