Let's break this down. We're going to add 2331 and 6135 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 3 & 3 in column 2 and get 6.

Step 3: We'll start by adding the digits 3 & 1 in column 3 and get 4.

Step 4: We'll start by adding the digits 2 & 6 in column 4 and get 8.

So, 2331 + 6135 = 8466.

