Let's break this down. We're going to add 126 and 3673 together.

Step 1: We'll start by adding the digits 6 & 3 in column 1 and get 9.

Step 2: We'll start by adding the digits 2 & 7 in column 2 and get 9.

Step 3: We'll start by adding the digits 1 & 6 in column 3 and get 7.

Step 4: We'll start by adding the digits 0 & 3 in column 4 and get 3.

So, 126 + 3673 = 3799.

