Let's break this down. We're going to add 6864 and 2123 together.

Step 1: We'll start by adding the digits 4 & 3 in column 1 and get 7.

Step 2: We'll start by adding the digits 6 & 2 in column 2 and get 8.

Step 3: We'll start by adding the digits 8 & 1 in column 3 and get 9.

Step 4: We'll start by adding the digits 6 & 2 in column 4 and get 8.

So, 6864 + 2123 = 8987.

