Okay, we are given 3420 and 2131. Let's add them up step by step.

Step 1: We'll start by adding the digits 0 & 1 in column 1 and get 1.

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 4 & 1 in column 3 and get 5.

Step 4: We'll start by adding the digits 3 & 2 in column 4 and get 5.

So, 3420 + 2131 = 5551.

