OK, let's do this. We've got 3012 and 87 and we're adding them all together.

Step 1: We'll start by adding the digits 2 & 7 in column 1 and get 9.

Step 2: We'll start by adding the digits 1 & 8 in column 2 and get 9.

Step 3: We'll start by adding the digits 0 & 0 in column 3 and get 0.

Step 4: We'll start by adding the digits 3 & 0 in column 4 and get 3.

So, 3012 + 87 = 3099.

