Ans: (a) $(9m+n)(9m-n)$ (b) $(9m-n)(9m+n+2)$
-
$\begin{array}{cl}
& 81m^2-n^2 \\
= & (9m-n)(9m+n)
\end{array}$ -
$\begin{array}{cl}
& 81m^2-n^2+18m-2n \\
= & (9m-n)(9m+n) +2(9m-n) \\
= & (9m-n)(9m+n+2)
\end{array}$
