\(x^2-xz-9y^2+3yz\)
\(=\)\(\left(x^2-9y^2\right)-\left(xz-3yz\right)\)
\(=\)\(\left(x-3y\right)\left(x+3y\right)-z\left(x-3y\right)\)
\(=\)\(\left(x-3y\right)\left(x+3y-z\right)\)
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\(x^2-xz-9y^2+3yz\)
\(=\left(x^2-9y^2\right)-\left(xz-3yz\right)\)
\(=\left(x-3y\right)\left(x+3y\right)-z\left(x-3y\right)\)
\(=\left(x-3y\right)\left(x+3y-z\right)\)