\(xyz-xy-yz-zx+x+y+z-1\)
\(=\left(xyz-xy\right)-\left(yz+zx\right)+\left(x+y\right)+\left(z-1\right)\)
\(=xy\left(z-1\right)-z\left(x+y\right)+\left(x+y\right)+\left(z-1\right)\)
\(=[xy\left(z-1\right)+\left(z-1\right)]-[z\left(x+y\right)-\left(x+y\right)]\)
\(=\left(z-1\right)\left(xy+1\right)-\left(x+y\right)\left(z-1\right)\)
\(=\left(z-1\right)[\left(xy+1\right)-\left(x+y\right)]\)
\(=\left(z-1\right)\left(xy+1-x-y\right)\)