OK, let's do this. We've got 1339 and 2603 and we're adding them all together.

Step 1: We'll start by adding the digits 9 & 3 in column 1 and get 12.
We'll write down the last digit 2 and carry the 1 to the next column.

Step 2: We'll start by adding the digits 3 & 0 in column 2 and get 4.

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

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

So, 1339 + 2603 = 3942.

