Answer:
(40 + 2x)feet
Explanation:
see attachment for the figure.
Let 'L' represents the length of the rectangular swimming pool i.e 40ft wide.
and pool is bordered by square tiles that are x feet in length surrounding the pool
therefore,
In order to find the longest side of the pool and the tile border, take the sum of length of the swimming pool i.e 40 adding two times the wide of the square tiles
(40+2x)feet.