Okay, we are given 215 and 604. Let's add them up step by step.

Step 1: We'll start by adding the digits 5 & 4 in column 1 and get 9.

Step 2: We'll start by adding the digits 1 & 0 in column 2 and get 1.

Step 3: We'll start by adding the digits 2 & 6 in column 3 and get 8.

So, 215 + 604 = 819.

