Đáp án:
$\begin{array}{l}
81{x^2} + 6yz - 9{y^2} - {z^2}\\
= {\left( {9x} \right)^2} - \left( {9{y^2} - 6yz + {z^2}} \right)\\
= {\left( {9x} \right)^2} - {\left( {3y - z} \right)^2}\\
= \left( {9x - 3y + z} \right)\left( {9x + 3y - z} \right)
\end{array}$