Alright, let's solve this problem step by step. We have 1866 and 7123 and we're adding them 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 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 1 & 7 in column 4 and get 8.

So, 1866 + 7123 = 8989.

