## Elementary Algebra

While $y^4-81$ can indeed be factored to $(y^2+9)(y^2-9)$, this is not the completely factored form of the expression. The factor $y^2-9$ is the difference of two perfect squares and can be further factored down to $(x-3)(x+3)$. This gives us a completely factored expression of $(x^2+9)(x+3)(x-3)$.