Let x be the side of the square pool
the area of the square pool is x^2

the new side of the pool will be (x+6) and (x-6)
so when you solve for the area of the rectangular square using difference of two squares the answer will be (x^2-36)

so in conclusion the new area was decreased by 36 units

area of the square-------x^2
area of the rectangle---x^2-36