We are presented with 867 and 5721. Let's work out the sum.

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 6 & 2 in column 2 and get 8.

Step 3: We'll start by adding the digits 8 & 7 in column 3 and get 15.
We'll write down the last digit 5 and carry the 1 to the next column.

Step 4: We'll start by adding the digits 0 & 5 in column 4 and get 6.

So, 867 + 5721 = 6588.

