Ans: (a) $(x-3y)^2$ (b) $(x-3y)(x-3y+7)$
-
$\begin{array}{cl}
& x^2-6xy+9y^2 \\
= & (x-3y)^2
\end{array}$ -
$\begin{array}{cl}
& x^2-6xy+9y^2+7x-21y \\
= & (x-3y)^2 +7(x-3y) \\
= & (x-3y)(x-3y+7)
\end{array}$
