OK, let's do this. We've got 339 and 109 and we're adding them all together.

Step 1: We'll start by adding the digits 9 & 9 in column 1 and get 18.
We'll write down the last digit 8 and carry the 1 to the next column.

Step 2: We'll start by adding the digits 3 & 0 in column 2 and get 4.

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

So, 339 + 109 = 448.

