Let's break this down. We're going to add 6121 and 3107 together.

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

Step 2: We'll start by adding the digits 2 & 0 in column 2 and get 2.

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

Step 4: We'll start by adding the digits 6 & 3 in column 4 and get 9.

So, 6121 + 3107 = 9228.

