Check the picture below.
so the deck is a 72x52 rectangular area, and the pool is simply two semicircles with a rectangle, as you see in the picture, we can put both semi-circles and make one circle with their area, so both together are really the area of a circle whose diameter is 29, and thus its radius is half that or 14.5.
Now, the area Belvedere is going to seal is really the "shaded" or bricked area, if we take the area of the whole deck, including the pool, and then get the area of the pool and subtract it from that of the deck's, what's leftover, is the area that wasn't subtracted, namely the "shaded" area.