OK, let's do this. We've got 3768 and 21 and we're adding them all together.

Step 1: We'll start by adding the digits 8 & 1 in column 1 and get 9.

Step 2: We'll start by adding the digits 6 & 2 in column 2 and get 8.

Step 3: We'll start by adding the digits 7 & 0 in column 3 and get 7.

Step 4: We'll start by adding the digits 3 & 0 in column 4 and get 3.

So, 3768 + 21 = 3789.

