No problem, let's work through this together. We're starting with 2487 and 6391 and adding them all up.

Step 1: We'll start by adding the digits 7 & 1 in column 1 and get 8.

Step 2: We'll start by adding the digits 8 & 9 in column 2 and get 17.
We'll write down the last digit 7 and carry the 1 to the next column.

Step 3: We'll start by adding the digits 4 & 3 in column 3 and get 8.

Step 4: We'll start by adding the digits 2 & 6 in column 4 and get 8.

So, 2487 + 6391 = 8878.

