Answer: 123 square feet
Step-by-step explanation: The question is a little odd, but I can break it down easily. It wants us to find the area of a three foot border around his room.
First, the whole area of his room is 20 x 24 = 480
Next, we have to make the measurements 3 feet smaller on each wall, so 17 x 21 = 357
Finally, to find the area of the border, we simply do 480 - 357 = 123
He needs to buy 123 square feet of carpeting
Hope this helps!