OK, let's do this. We've got 6121 and 1815 and we're adding them all together.

Step 1: We'll start by adding the digits 1 & 5 in column 1 and get 6.

Step 2: We'll start by adding the digits 2 & 1 in column 2 and get 3.

Step 3: We'll start by adding the digits 1 & 8 in column 3 and get 9.

Step 4: We'll start by adding the digits 6 & 1 in column 4 and get 7.

So, 6121 + 1815 = 7936.

