Okay, we are given 3921 and 2552. Let's add them up step by step.

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

Step 2: We'll start by adding the digits 2 & 5 in column 2 and get 7.

Step 3: We'll start by adding the digits 9 & 5 in column 3 and get 14.
We'll write down the last digit 4 and carry the 1 to the next column.

Step 4: We'll start by adding the digits 3 & 2 in column 4 and get 6.

So, 3921 + 2552 = 6473.

