We've got two numbers: 3591 and 6306. Let's find their sum.

Step 1: We'll start by adding the digits 1 & 6 in column 1 and get 7.

Step 2: We'll start by adding the digits 9 & 0 in column 2 and get 9.

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

Step 4: We'll start by adding the digits 3 & 6 in column 4 and get 9.

So, 3591 + 6306 = 9897.

