Let's break this down. We're going to add 3900 and 6039 together.

Step 1: We'll start by adding the digits 0 & 9 in column 1 and get 9.

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

Step 3: We'll start by adding the digits 9 & 0 in column 3 and get 9.

Step 4: We'll start by adding the digits 3 & 6 in column 4 and get 9.

So, 3900 + 6039 = 9939.

