Alright, let's solve this problem step by step. We have 621 and 313 and we're adding them together.

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

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

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

So, 621 + 313 = 934.

