6: =x^2-7xy+5xy-35y^2
=x(x-7y)+5y(x-7y)
=(x-7y)(x+5y)
7: =x^2-2xy-8xy+16y^2
=x(x-2y)-8y(x-2y)
=(x-2y)(x-8y)
8: =3x^2-6xy-4xy+8y^2
=3x(x-2y)-4y(x-2y)
=(x-2y)(3x-4y)
9: =4x^2+4xy+y^2-16y^2
=(2x+y)^2-16y^2
=(2x+y-4y)(2x+y+4y)
=(2x-3y)*(2x+5y)
10: =2(x^2+5xy+4y^2)
=2(x+y)(x+4y)
11: =5x(x+2y+y^2)