Answer
$4g^{2}-81h^{2}$
Work Step by Step
$(2g+9h)(2g-9h)$
...identify which terms correspond to $a $ and $b $
in the rule for the product of a sum and difference ($(a+b)(a-b)=a^{2}-b^{2}$).
$a=2g,\ b=9h$
...substitute for $a $ and $b $ in the rule
$(2g+9h)(2g-9h)=(2g)^{2}-(9h)^{2}$
...simplify.
=$4g^{2}-81h^{2}$
