Let's break this down. We're going to add 1521 and 2007 together.

Step 1: We'll start by adding the digits 1 & 7 in column 1 and get 8.

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

Step 3: We'll start by adding the digits 5 & 0 in column 3 and get 5.

Step 4: We'll start by adding the digits 1 & 2 in column 4 and get 3.

So, 1521 + 2007 = 3528.

