Answer
$(h+k+3)(h+k-3)$
Work Step by Step
$\bf{\text{Solution Outline:}}$
To factor the given expression, $
(h+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 difference of $2$ squares, the factored form of the expression above is
\begin{array}{l}\require{cancel}
[(h+k)+3][(h+k)-3]
\\\\=
(h+k+3)(h+k-3)
.\end{array}