1) x^6-y^3=(x^2)^3-y^3=(x^2-y)((x^2)^2+x^2y+y^2)=(x^2-y)((x^4+x^2y+y^2)
2) (a-1)^3-(2a-9)^3=((a-1)-(2a-9))((a-1)^2+(a-1)(2a-9)+(2a-9)^2)=(a-1-2a+9)(a^2-2a+1+2a^2-9a-2a+9+4a^2-36a+81)=(-a+8)(7a^2-49a+91)=7(-a+8)(a^2-7a+13)
