Question 41209
What you have found out is the combined area of the deck and the pool.
The area of only deck = (combined area of the deck and the pool) - (area of pool)
= {{{(4d^2+120d+800) - (800)}}}
= {{{4d^2+120d}}}


Note this is true only when the length of the two rectangles are parallel.
This means that the deck is of constant width 'd'.
Else, if the length (longer side) of the outer rectangle is not parallel to the length (longer side) of the inner rectangle for then you cannot write
"The length of the large rectangle is 20+2d
The width of the large rectangle is 40+2d"


Understand?