We are presented with 6540 and 2109. Let's work out the sum.

Step 1: We'll start by adding the digits 0 & 9 in column 1 and get 9.

Step 2: We'll start by adding the digits 4 & 0 in column 2 and get 4.

Step 3: We'll start by adding the digits 5 & 1 in column 3 and get 6.

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

So, 6540 + 2109 = 8649.

