Let's break this down. We're going to add 255 and 3121 together.

Step 1: We'll start by adding the digits 5 & 1 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 2 & 1 in column 3 and get 3.

Step 4: We'll start by adding the digits 0 & 3 in column 4 and get 3.

So, 255 + 3121 = 3376.

