Let's solve this addition problem. We have 243 and 423, and we need to add them together.

Step 1: We'll start by adding the digits 3 & 3 in column 1 and get 6.

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

Step 3: We'll start by adding the digits 2 & 4 in column 3 and get 6.

So, 243 + 423 = 666.

