Okay, we are given 2421 and 1572. Let's add them up step by step.

Step 1: We'll start by adding the digits 1 & 2 in column 1 and get 3.

Step 2: We'll start by adding the digits 2 & 7 in column 2 and get 9.

Step 3: We'll start by adding the digits 4 & 5 in column 3 and get 9.

Step 4: We'll start by adding the digits 2 & 1 in column 4 and get 3.

So, 2421 + 1572 = 3993.

