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