Let's break this down. We're going to add 2521 and 3707 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 & 7 in column 3 and get 12.
We'll write down the last digit 2 and carry the 1 to the next column.

Step 4: We'll start by adding the digits 2 & 3 in column 4 and get 6.

So, 2521 + 3707 = 6228.

