Let's break this down. We're going to add 3357 and 3331 together.

Step 1: We'll start by adding the digits 7 & 1 in column 1 and get 8.

Step 2: We'll start by adding the digits 5 & 3 in column 2 and get 8.

Step 3: We'll start by adding the digits 3 & 3 in column 3 and get 6.

Step 4: We'll start by adding the digits 3 & 3 in column 4 and get 6.

So, 3357 + 3331 = 6688.

