A swimming pool is 50 feet by 150 feet.

Two  sides  of the decking  wil be rectangles with the dimensions [ (150 + 6) * 3]

And the other two sides will be rectangles  with the dimensions  ( 50 * 3)

So  the total area of the decking is

2 [  (150 + 6) * 3 ]  +  2 (50 * 3)  =

2 ( 156 * 3)  + 2 ( 150)  =

2 ( 468)  + 2 (150)  =

2 ( 468 +150)  =

2 ( 618)  =

1236  ft^2

[Area of the swimming pool + deck] - [Area of the swimming pool only] = Area of the deck.

[ 156            x                56                     ] - [   150              x                50            ] =

                  8,736                                       -                      7,500                               =1,236 feet^2