Answer
The length of a side of a square is $3n+9$.
Work Step by Step
$9n^{2}+54n+81=$
...write first and last terms as squares.
$=(3n)^{2}+54n+9^{2}$
...does middle term equal $2ab$?
$54n=2(3n)(9)$
yes.
$=(3n)^{2}+2(3n)(9)+9^{2}$
...write as the square of a binomial.
$=(3n+9)^{2}$
The length of a side of a square is $3n+9$.
