Okay, we are given 8706 and 8421. Let's add them up step by step.

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

Step 2: We'll start by adding the digits 0 & 2 in column 2 and get 2.

Step 3: We'll start by adding the digits 7 & 4 in column 3 and get 11.
We'll write down the last digit 1 and carry the 1 to the next column.

Step 4: We'll start by adding the digits 8 & 8 in column 4 and get 17.
We'll write down the last digit 7 and carry the 1 to the next column.

So, 8706 + 8421 = 17127.

