Okay, we are given 2709 and 921. Let's add them up step by step.

Step 1: We'll start by adding the digits 9 & 1 in column 1 and get 10.
We'll write down the last digit 0 and carry the 1 to the next column.

Step 2: We'll start by adding the digits 0 & 2 in column 2 and get 3.

Step 3: We'll start by adding the digits 7 & 9 in column 3 and get 16.
We'll write down the last digit 6 and carry the 1 to the next column.

Step 4: We'll start by adding the digits 2 & 0 in column 4 and get 3.

So, 2709 + 921 = 3630.

