Answer
The side length of the square is
$8r-9$.
Work Step by Step
$64r^{2}-144r+81=$
...write first and last terms as squares.
$=(8r)^{2}-144r+(9)^{2}$
...does middle term equal $2ab$?
$144r=2\cdot(8r)\cdot(9)$
$=(8r)^{2}-2\cdot(8r)\cdot(9)+(9)^{2}$
...write as the square of a binomial.
$=(8r-9)^{2}$
The side length of the square is
$8r-9$.
