OK, let's do this. We've got 876978 and 21677, and we're subtracting the second number from the first.

Step 1: We'll start by subtracting the digit 7 and the borrow 0 from 8 in column 1 and get 1.
1 is the first digit of our result.

Step 2: We'll start by subtracting the digit 7 and the borrow 0 from 7 in column 2 and get 0.
0 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 6 in column 4 and get 5.
5 is the next digit of our result.

Step 5: We'll start by subtracting the digit 2 and the borrow 0 from 7 in column 5 and get 5.
5 is the next digit of our result.

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

So, 876978 - 21677 = 855301.