If you followed all the steps apart from the last one there will be three options remaining: 64, 84, and 86. You then had to add up the digits, 64=6+4=10, 84=8+4=12, and 86=8+6=14. Finally, you then had to take up the middle biggest number (12) and put it back as it was before the digits were added together and your answer should be 84.
This one is fairly easy if you use elimination if you follow all the first 5 steps you get three options: 525, 327, and 723 but if you followed the last step you would reach your answer. The answer was 327.