Answer
$(k+3)(k-3)$
Work Step by Step
$\bf{\text{Solution Outline:}}$
To factor the given expression, $
k^2-9
,$ use the factoring of the difference of $2$ squares.
$\bf{\text{Solution Details:}}$
Using $a^2-b^2=(a+b)(a-b)$ or the factoring of the diffrence of $2$ squares, the factored form of the expression above is
\begin{array}{l}\require{cancel}
(k+3)(k-3)
.\end{array}