OK, let's do this. We've got 5390 and 3609 and we're adding them all together.

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 9 & 0 in column 2 and get 9.

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

Step 4: We'll start by adding the digits 5 & 3 in column 4 and get 8.

So, 5390 + 3609 = 8999.

