50x^2(x-2y)^2-2x^2(2y-z)^2
=2x^2[25(x-2y)^2-(2y-z)^2]
=2x^2[5(x-2y)+(2y-z)][5(x-2y)-(2y-z)]
=2x^2(5x--8y-z)(5x-12y+z)