Okay, we are given 6721 and 8911. Let's add them up step by step.

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 2 & 1 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 6 & 8 in column 4 and get 15.
We'll write down the last digit 5 and carry the 1 to the next column.

So, 6721 + 8911 = 15632.

