Answer
$(y+z+9)(y+z-9)$
Work Step by Step
$\bf{\text{Solution Outline:}}$
To factor the given expression, $
(y+z)^2-81
,$ 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}
[(y+z)+9][(y+z)-9]
\\\\=
(y+z+9)(y+z-9)
.\end{array}