We can solve this together! We're beginning with 91721 and removing 91709 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 0 and the borrow 1 from 2 in column 2 and get 1.
1 is the next digit of our result.

Step 3: We'll start by subtracting the digit 7 and the borrow 0 from 7 in column 3 and get 0.
0 is the next digit of our result.

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

Step 5: We'll start by subtracting the digit 9 and the borrow 0 from 9 in column 5 and get 0.
0 is the next digit of our result.

So, 91721 - 91709 = 12.