a. \(x^2-2xy+y^2-xz+yz=\left(x^2-2xy+y^2\right)-\left(xz-yz\right)=\left(x-y\right)^2-z\left(x-y\right)=\left(x-y\right)\left(x-y-z\right)\)
c. \(x^2-9x+18=x^2-3x-6x+18=\left(x^2-3x\right)-\left(6x-18\right)=x\left(x-3\right)-6\left(x-3\right)=\left(x-3\right)\left(x-6\right)\)