Alright, let's solve this problem step by step. We have 5123 and 1213 and we're adding them together.

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

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 1 & 2 in column 3 and get 3.

Step 4: We'll start by adding the digits 5 & 1 in column 4 and get 6.

So, 5123 + 1213 = 6336.

