The area of the deck is 50.4 x 22 = 1108.8 square ft.
The area of 1 cut out is 4.5 x 4.5 = 20.25
There are 3 cut outs so total area of the cut outs is 20.25 x 3 = 60.75 square ft.
The total area of the boards is : 1108.8 - 60.75 = 1048.05 square feet.