OK, let's do this. We've got 2391 and 3570 and we're adding them all together.

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 9 & 7 in column 2 and get 16.
We'll write down the last digit 6 and carry the 1 to the next column.

Step 3: We'll start by adding the digits 3 & 5 in column 3 and get 9.

Step 4: We'll start by adding the digits 2 & 3 in column 4 and get 5.

So, 2391 + 3570 = 5961.

