Let's break this down. We're going to add 3851 and 4125 together.

Step 1: We'll start by adding the digits 1 & 5 in column 1 and get 6.

Step 2: We'll start by adding the digits 5 & 2 in column 2 and get 7.

Step 3: We'll start by adding the digits 8 & 1 in column 3 and get 9.

Step 4: We'll start by adding the digits 3 & 4 in column 4 and get 7.

So, 3851 + 4125 = 7976.

