OK, let's do this. We've got 6243 and 3151 and we're adding them all together.

Step 1: We'll start by adding the digits 3 & 1 in column 1 and get 4.

Step 2: We'll start by adding the digits 4 & 5 in column 2 and get 9.

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

Step 4: We'll start by adding the digits 6 & 3 in column 4 and get 9.

So, 6243 + 3151 = 9394.

