Answer
$r^{4}-9s^{2}$
Work Step by Step
$(r^{2}+3s)(r^{2}-3s)$
...identify which terms correspond to $a $ and $b $
in the rule for the product of a sum and difference ($(a+b)(a-b)=a^{2}-b^{2}$).
$a=r^{2},\ b=3s$
...substitute for $a $ and $b $ in the rule
$(r^{2}+3s)(r^{2}-3s)=(r^{2})^{2}-(3s)^{2}$
...simplify.
=$r^{4}-9s^{2}$
