No problem, let's work through this together. We're starting with 7821 and 6900 and adding them all up.

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

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

Step 3: We'll start by adding the digits 8 & 9 in column 3 and get 17.
We'll write down the last digit 7 and carry the 1 to the next column.

Step 4: We'll start by adding the digits 7 & 6 in column 4 and get 14.
We'll write down the last digit 4 and carry the 1 to the next column.

So, 7821 + 6900 = 14721.

