Okay, we are given 6590 and 399. Let's add them up step by step.

Step 1: We'll start by adding the digits 0 & 9 in column 1 and get 9.

Step 2: We'll start by adding the digits 9 & 9 in column 2 and get 18.
We'll write down the last digit 8 and carry the 1 to the next column.

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

Step 4: We'll start by adding the digits 6 & 0 in column 4 and get 6.

So, 6590 + 399 = 6989.

