We can solve this together! We're beginning with 270903 and removing 99399 from it.

Step 1: We'll start by subtracting the digit 9 and the borrow 0 from 3 in column 1 and get -6.
We add -6 to 10 and get 4 as the first digit of the result.

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

Step 3: We'll start by subtracting the digit 3 and the borrow 1 from 9 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 0 in column 4 and get -9.
We add -9 to 10 and get 1 as the next digit of the result.

Step 5: We'll start by subtracting the digit 9 and the borrow 1 from 7 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 2 in column 6 and get 1.
1 is the next digit of our result.

So, 270903 - 99399 = 171504.