OK, let's do this. We've got 3964 and 8431 and we're adding them all together.

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

Step 2: We'll start by adding the digits 6 & 3 in column 2 and get 9.

Step 3: We'll start by adding the digits 9 & 4 in column 3 and get 13.
We'll write down the last digit 3 and carry the 1 to the next column.

Step 4: We'll start by adding the digits 3 & 8 in column 4 and get 12.
We'll write down the last digit 2 and carry the 1 to the next column.

So, 3964 + 8431 = 12395.

