Okay, we are tasked with adding 8813 and 3963. Let's begin!

Step 1: We'll start by adding the digits 3 & 3 in column 1 and get 6.

Step 2: We'll start by adding the digits 1 & 6 in column 2 and get 7.

Step 3: We'll start by adding the digits 8 & 9 in column 3 and get 17.
We'll write down the last digit 7 and carry the 1 to the next column.

Step 4: We'll start by adding the digits 8 & 3 in column 4 and get 12.
We'll write down the last digit 2 and carry the 1 to the next column.

So, 8813 + 3963 = 12776.

