Let's break this down. We're going to add 863 and 903 together.

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

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

Step 3: We'll start by adding the digits 8 & 9 in column 3 and get 17.
We'll write down the last digit 7 and carry the 1 to the next column.

So, 863 + 903 = 1766.

