Ans: (a) $(x-3y)(x-y)$ (b) $(x-3y)(x-y+11)$
-
$\begin{array}{cl}
& x^2 -4xy+3y^2 \\
& (x-3y)(x-y)
\end{array}$ -
$\begin{array}{cl}
& x^2 -4xy +3y^2 +11x -33y \\
= & (x-3y)(x-y) + 11(x-3y) \\
= & (x-3y)(x-y +11)
\end{array}$
