\(1,3x^2-6xy+3y^2-12z^2\)
\(=3\left(x^2-2xy+y^2-4z^2\right)\)
\(=3[\left(x^2-2xy+y^2\right)-4z^2]\)
\(=3\left[\left(x-y\right)^2-4z^2\right]\)
\(=3\left(x-y-2x\right)\left(x-y+2z\right)\)
\(2,x^2-2x-15\)
\(=x^2-5x+3x-15\)
\(=\left(x^2-5x\right)+\left(3x-15\right)\)
\(=x\left(x-5\right)+3\left(x-5\right)\)
\(=\left(x+3\right)\left(x-5\right)\)