You can 
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The outside dimensions of the large rectangle are  30+2x  by  40+2x  feet
and its area is the product  (30+2x)*(40+2x)  square feet.
The pool's dimensions are 30  by  40  feet, and its area is 30*40 square feet.
The area of the pathway is the DIFFERENCE of these areas
    (30+2x)*(40+2x) - 30*40.
It is the expression you are looking for.
Solved, answered and explained.
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