Answer
$16x^{2}+160x+400$.
Work Step by Step
The side length of the expanded deck is $4(x+5)=4x+20$
The area of the expanded deck is $(4x+20)^{2}$
...square the binomial.
$=(4x)^{2}+2(4x)(20)+20^{2}$
...simplify.
$=16x^{2}+160x+400$
The area of the expanded deck is $16x^{2}+160x+400$.
