Let's break this down. We're going to add 9963 and 521 together.

Step 1: We'll start by adding the digits 3 & 1 in column 1 and get 4.

Step 2: We'll start by adding the digits 6 & 2 in column 2 and get 8.

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 9 & 0 in column 4 and get 10.
We'll write down the last digit 0 and carry the 1 to the next column.

So, 9963 + 521 = 10484.

