The area of the only deck is 528 ft²
<u><em>Explanation</em></u>
The length and width of the swimming pool are 20 ft and 12 ft respectively.
So, <u>the area of the swimming pool</u> 
Now a deck is being built around the pool and it has a uniform width of 6 ft.
So, the length of the pool including the deck
and the width of the pool including the deck 
Thus, <u>the area of the pool including the deck</u> 
So, the area of the deck only