Answer
$(8r^{3}-9)^{2}$
Work Step by Step
$64r^{6}-144r^{3}+81=$
...write as the square of a binomial.
...($a=8r^{3},\ b=9,)\ \quad $test the middle term: $2ab=2(8r^{3})(9)=144r^3$)
$=(8r^{3}-9)^{2}$
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