Okay, we are given 3390 and 3407. Let's add them up step by step.

Step 1: We'll start by adding the digits 0 & 7 in column 1 and get 7.

Step 2: We'll start by adding the digits 9 & 0 in column 2 and get 9.

Step 3: We'll start by adding the digits 3 & 4 in column 3 and get 7.

Step 4: We'll start by adding the digits 3 & 3 in column 4 and get 6.

So, 3390 + 3407 = 6797.

