Alright, let's solve this problem step by step. We have 6123 and 3750 and we're adding them together.

Step 1: We'll start by adding the digits 3 & 0 in column 1 and get 3.

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

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

Step 4: We'll start by adding the digits 6 & 3 in column 4 and get 9.

So, 6123 + 3750 = 9873.

