OK, let's do this. We've got 7407 and 3209 and we're adding them all together.

Step 1: We'll start by adding the digits 7 & 9 in column 1 and get 16.
We'll write down the last digit 6 and carry the 1 to the next column.

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

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

Step 4: We'll start by adding the digits 7 & 3 in column 4 and get 10.
We'll write down the last digit 0 and carry the 1 to the next column.

So, 7407 + 3209 = 10616.

