We have numbers 215 and 128. Let's start adding them together.

Step 1: We'll start by adding the digits 5 & 8 in column 1 and get 13.
We'll write down the last digit 3 and carry the 1 to the next column.

Step 2: We'll start by adding the digits 1 & 2 in column 2 and get 4.

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

So, 215 + 128 = 343.

