OK, let's do this. We've got 3129 and 6186 and we're adding them all together.

Step 1: We'll start by adding the digits 9 & 6 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 2 & 8 in column 2 and get 11.
We'll write down the last digit 1 and carry the 1 to the next column.

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

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

So, 3129 + 6186 = 9315.

