Alright, let's solve this problem step by step. We have 755 and 8123 and we're adding them together.

Step 1: We'll start by adding the digits 5 & 3 in column 1 and get 8.

Step 2: We'll start by adding the digits 5 & 2 in column 2 and get 7.

Step 3: We'll start by adding the digits 7 & 1 in column 3 and get 8.

Step 4: We'll start by adding the digits 0 & 8 in column 4 and get 8.

So, 755 + 8123 = 8878.

