Okay, we are tasked with adding 1399 and 3537. Let's begin!

Step 1: We'll start by adding the digits 9 & 7 in column 1 and get 16.
We'll write down the last digit 6 and carry the 1 to the next column.

Step 2: We'll start by adding the digits 9 & 3 in column 2 and get 13.
We'll write down the last digit 3 and carry the 1 to the next column.

Step 3: We'll start by adding the digits 3 & 5 in column 3 and get 9.

Step 4: We'll start by adding the digits 1 & 3 in column 4 and get 4.

So, 1399 + 3537 = 4936.

