Okay, we are tasked with adding 3663 and 3207. Let's begin!

Step 1: We'll start by adding the digits 3 & 7 in column 1 and get 10.
We'll write down the last digit 0 and carry the 1 to the next column.

Step 2: We'll start by adding the digits 6 & 0 in column 2 and get 7.

Step 3: We'll start by adding the digits 6 & 2 in column 3 and get 8.

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

So, 3663 + 3207 = 6870.

