Answer
$r^{6}-36t^{8}$
Work Step by Step
Recall: $(a-b)(a+b)=a^{2}-b^{2}$
$\implies (r^{3}-6t^{4})(r^{3}+6t^{4})=(r^{3})^{2}-(6t^{4})^{2}$
$=r^{6}-36t^{8}$
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