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