Okay, let's tackle this math problem. We're starting with 667640 and subtracting 29128.

Step 1: We'll start by subtracting the digit 8 and the borrow 0 from 0 in column 1 and get -8.
We add -8 to 10 and get 2 as the first digit of the result.

Step 2: We'll start by subtracting the digit 2 and the borrow 1 from 4 in column 2 and get 1.
1 is the next digit of our result.

Step 3: We'll start by subtracting the digit 1 and the borrow 0 from 6 in column 3 and get 5.
5 is the next digit of our result.

Step 4: We'll start by subtracting the digit 9 and the borrow 0 from 7 in column 4 and get -2.
We add -2 to 10 and get 8 as the next digit of the result.

Step 5: We'll start by subtracting the digit 2 and the borrow 1 from 6 in column 5 and get 3.
3 is the next digit of our result.

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

So, 667640 - 29128 = 638512.