OK, let's do this. We've got 3935 and 1721 and we're adding them all together.

Step 1: We'll start by adding the digits 5 & 1 in column 1 and get 6.

Step 2: We'll start by adding the digits 3 & 2 in column 2 and get 5.

Step 3: We'll start by adding the digits 9 & 7 in column 3 and get 16.
We'll write down the last digit 6 and carry the 1 to the next column.

Step 4: We'll start by adding the digits 3 & 1 in column 4 and get 5.

So, 3935 + 1721 = 5656.

