Okay, let's tackle this math problem. We're starting with 79240 and subtracting 58138.

Step 1: We'll start by subtracting the digit 8 and the borrow 0 from 0 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 3 and the borrow 1 from 4 in column 2 and get 0.
0 is the next digit of our result.

Step 3: We'll start by subtracting the digit 1 and the borrow 0 from 2 in column 3 and get 1.
1 is the next digit of our result.

Step 4: We'll start by subtracting the digit 8 and the borrow 0 from 9 in column 4 and get 1.
1 is the next digit of our result.

Step 5: We'll start by subtracting the digit 5 and the borrow 0 from 7 in column 5 and get 2.
2 is the next digit of our result.

So, 79240 - 58138 = 21102.