x^6-y^6
fist look at as a difference of squares:
(x^3)2 - (y^3)2
=(x^3 + y^3)(x^3 - y^3)
Now you have the sum of cubes and the difference of cubes, I assume you know the formulas for those. So you can keep going
(x+y)(x^2-xy+y^2)(x-y)(x^2+xy+y^2) would be your final answer.