Let's break this down. We're going to add 4390 and 4401 together.

Step 1: We'll start by adding the digits 0 & 1 in column 1 and get 1.

Step 2: We'll start by adding the digits 9 & 0 in column 2 and get 9.

Step 3: We'll start by adding the digits 3 & 4 in column 3 and get 7.

Step 4: We'll start by adding the digits 4 & 4 in column 4 and get 8.

So, 4390 + 4401 = 8791.

