Okay, let's tackle this math problem. We're starting with 76721 and subtracting 67998.

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

Step 2: We'll start by subtracting the digit 9 and the borrow 1 from 2 in column 2 and get -8.
We add -8 to 10 and get 2 as the next digit of the result.

Step 3: We'll start by subtracting the digit 9 and the borrow 1 from 7 in column 3 and get -3.
We add -3 to 10 and get 7 as the next digit of the result.

Step 4: We'll start by subtracting the digit 7 and the borrow 1 from 6 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 6 and the borrow 1 from 7 in column 5 and get 0.
0 is the next digit of our result.

So, 76721 - 67998 = 8723.