Okay, we are tasked with adding 2101 and 9121. Let's begin!

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

Step 2: We'll start by adding the digits 0 & 2 in column 2 and get 2.

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

Step 4: We'll start by adding the digits 2 & 9 in column 4 and get 11.
We'll write down the last digit 1 and carry the 1 to the next column.

So, 2101 + 9121 = 11222.

