Here we go! We're going to add 3599 and 6994 together.

Step 1: We'll start by adding the digits 9 & 4 in column 1 and get 13.
We'll write down the last digit 3 and carry the 1 to the next column.

Step 2: We'll start by adding the digits 9 & 9 in column 2 and get 19.
We'll write down the last digit 9 and carry the 1 to the next column.

Step 3: We'll start by adding the digits 5 & 9 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 3 & 6 in column 4 and get 10.
We'll write down the last digit 0 and carry the 1 to the next column.

So, 3599 + 6994 = 10593.

