OK, let's do this. We've got 3121 and 6885 and we're adding them all together.

Step 1: We'll start by adding the digits 1 & 5 in column 1 and get 6.

Step 2: We'll start by adding the digits 2 & 8 in column 2 and get 10.
We'll write down the last digit 0 and carry the 1 to the next column.

Step 3: We'll start by adding the digits 1 & 8 in column 3 and get 10.
We'll write down the last digit 0 and carry the 1 to the next column.

Step 4: We'll start by adding the digits 3 & 6 in column 4 and get 10.
We'll write down the last digit 0 and carry the 1 to the next column.

So, 3121 + 6885 = 10006.

