We've got two numbers: 6408 and 3407. Let's find their sum.

Step 1: We'll start by adding the digits 8 & 7 in column 1 and get 15.
We'll write down the last digit 5 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 & 4 in column 3 and get 8.

Step 4: We'll start by adding the digits 6 & 3 in column 4 and get 9.

So, 6408 + 3407 = 9815.

