Alright, let's solve this problem step by step. We have 621 and 168 and we're adding them together.

Step 1: We'll start by adding the digits 1 & 8 in column 1 and get 9.

Step 2: We'll start by adding the digits 2 & 6 in column 2 and get 8.

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

So, 621 + 168 = 789.

