We can solve this together! We're beginning with 840711 and removing 67899 from it.

Step 1: We'll start by subtracting the digit 9 and the borrow 0 from 1 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 9 and the borrow 1 from 1 in column 2 and get -9.
We add -9 to 10 and get 1 as the next digit of the result.

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

Step 4: We'll start by subtracting the digit 7 and the borrow 1 from 0 in column 4 and get -8.
We add -8 to 10 and get 2 as the next digit of the result.

Step 5: We'll start by subtracting the digit 6 and the borrow 1 from 4 in column 5 and get -3.
We add -3 to 10 and get 7 as the next digit of the result.

Step 6: We'll start by subtracting the digit 0 and the borrow 1 from 8 in column 6 and get 7.
7 is the next digit of our result.

So, 840711 - 67899 = 772812.