(x^2-7x)^2+6x-42
=[x(x-7)]^2+6[x-7]
=[x-7][x^2(x-7)+6 ]
=[x-7][x^3-7x^2+6]
=[x-7][x^2(x-1)-6(x^2-1)]
=[x-7][x^2(x-1)-6(x+1)(x-1)]
=[x-7][(x-1)(x^2-6x-6)]
25x^2-16y^2-4z^2+16yz
=25x^2-4[4y^2-4yz+z^2]
=25x^2-4[2y-z]^2
=[5x+2(2y-z)][5x-2(2y-z)]
=[5x+4y-2z][5x-4y+2z]