Let's get this math done. We have 3870 and 2821 and we're going to add them all together.

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 7 & 2 in column 2 and get 9.

Step 3: We'll start by adding the digits 8 & 8 in column 3 and get 16.
We'll write down the last digit 6 and carry the 1 to the next column.

Step 4: We'll start by adding the digits 3 & 2 in column 4 and get 6.

So, 3870 + 2821 = 6691.

