Okay, let's tackle this math problem. We're starting with 61958 and subtracting 21966.

Step 1: We'll start by subtracting the digit 6 and the borrow 0 from 8 in column 1 and get 2.
2 is the first digit of our result.

Step 2: We'll start by subtracting the digit 6 and the borrow 0 from 5 in column 2 and get -1.
We add -1 to 10 and get 9 as the next digit of the result.

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

Step 4: We'll start by subtracting the digit 1 and the borrow 1 from 1 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 2 and the borrow 1 from 6 in column 5 and get 3.
3 is the next digit of our result.

So, 61958 - 21966 = 39992.