Ans: (a) $(3x-7y)^2$ (b) $(3x-7y)(3x-7y-2)$
-
$\begin{array}{cl}
& 9x^2-42xy+49y^2 \\
= & (3x-7y)^2
\end{array}$ -
$\begin{array}{cl}
& 9x^2-42xy+49y^2-6x+14y \\
= & (3x-7y)^2 -2(3x-7y) \\
= & (3x-7y)(3x-7y-2)
\end{array}$
