Okay, let's tackle this math problem. We're starting with 99910086 and subtracting 21070939.

Step 1: We'll start by subtracting the digit 9 and the borrow 0 from 6 in column 1 and get -3.
We add -3 to 10 and get 7 as the first digit of the result.

Step 2: We'll start by subtracting the digit 3 and the borrow 1 from 8 in column 2 and get 4.
4 is the next digit of our result.

Step 3: We'll start by subtracting the digit 9 and the borrow 0 from 0 in column 3 and get -9.
We add -9 to 10 and get 1 as the next digit of the result.

Step 4: We'll start by subtracting the digit 0 and the borrow 1 from 0 in column 4 and get -1.
We add -1 to 10 and get 9 as the next digit of the result.

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

Step 6: We'll start by subtracting the digit 0 and the borrow 1 from 9 in column 6 and get 8.
8 is the next digit of our result.

Step 7: We'll start by subtracting the digit 1 and the borrow 0 from 9 in column 7 and get 8.
8 is the next digit of our result.

Step 8: We'll start by subtracting the digit 2 and the borrow 0 from 9 in column 8 and get 7.
7 is the next digit of our result.

So, 99910086 - 21070939 = 78839147.