Okay, we are tasked with adding 6121 and 538. Let's begin!

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 & 3 in column 2 and get 5.

Step 3: We'll start by adding the digits 1 & 5 in column 3 and get 6.

Step 4: We'll start by adding the digits 6 & 0 in column 4 and get 6.

So, 6121 + 538 = 6659.

