Let's break this down. We're going to add 2091 and 21 together.

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 9 & 2 in column 2 and get 11.
We'll write down the last digit 1 and carry the 1 to the next column.

Step 3: We'll start by adding the digits 0 & 0 in column 3 and get 1.

Step 4: We'll start by adding the digits 2 & 0 in column 4 and get 2.

So, 2091 + 21 = 2112.

