We can see that our polynomial is equal to
[tex]y^6-256y^2=y^2(y^4-256)[/tex]Now, we can factorize the term in parenthesis as
[tex]y^4-256=(y^2+16)(y^2-16)[/tex]Similarly,
[tex]y^2-16=(y+4)(y-4)[/tex]finally, we obtain
[tex]y^6-256y^2=y^2(y^2+16)(y+4)(y-4)[/tex]