OK, let's do this. We've got 3123 and 2865 and we're adding them all together.

Step 1: We'll start by adding the digits 3 & 5 in column 1 and get 8.

Step 2: We'll start by adding the digits 2 & 6 in column 2 and get 8.

Step 3: We'll start by adding the digits 1 & 8 in column 3 and get 9.

Step 4: We'll start by adding the digits 3 & 2 in column 4 and get 5.

So, 3123 + 2865 = 5988.

