Let's break this down. We're going to add 3123 and 1621 together.

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

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

Step 3: We'll start by adding the digits 1 & 6 in column 3 and get 7.

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

So, 3123 + 1621 = 4744.

