Answer
$g^{2}-100$
Work Step by Step
$(g+10)(g-10)=$
...identify which terms correspond to $a $ and $b $
...in the rule for the product of a sum and difference ($(a+b)(a-b)=a^{2}-b^{2}$)
$a=g,\ b=10$
...substitute for $a $and $b $ in the rule
$(g+10)(g-10)=g^{2}-10^{2}$
...simplify.
=$g^{2}-100$
