Okay, we are tasked with adding 6121 and 3935. Let's begin!

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

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 & 9 in column 3 and get 10.
We'll write down the last digit 0 and carry the 1 to the next column.

Step 4: We'll start by adding the digits 6 & 3 in column 4 and get 10.
We'll write down the last digit 0 and carry the 1 to the next column.

So, 6121 + 3935 = 10056.

