Consider $16a^{8}-81y^{8}$. Rewrite $16a^{8}-81y^{8}$ as $\left(4a^{4}\right)^{2}-\left(9y^{4}\right)^{2}$. The difference of squares can be factored using the rule: $p^{2}-q^{2}=\left(p-q\right)\left(p+q\right)$.
Consider $-9y^{4}+4a^{4}$. Rewrite $-9y^{4}+4a^{4}$ as $\left(2a^{2}\right)^{2}-\left(3y^{2}\right)^{2}$. The difference of squares can be factored using the rule: $p^{2}-q^{2}=\left(p-q\right)\left(p+q\right)$.
