Let's break this down. We're going to add 3921 and 33 together.

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

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

Step 3: We'll start by adding the digits 9 & 0 in column 3 and get 9.

Step 4: We'll start by adding the digits 3 & 0 in column 4 and get 3.

So, 3921 + 33 = 3954.

