OK, let's do this. We've got 3869 and 2421, and we're subtracting the second number from the first.

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

Step 2: We'll start by subtracting the digit 2 and the borrow 0 from 6 in column 2 and get 4.
4 is the next digit of our result.

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

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

So, 3869 - 2421 = 1448.