Let's solve this addition problem. We have 123 and 312, and we need to add them together.

Step 1: We'll start by adding the digits 3 & 2 in column 1 and get 5.

Step 2: We'll start by adding the digits 2 & 1 in column 2 and get 3.

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

So, 123 + 312 = 435.

