Question: Simplify:

$$\frac{\frac{16x^4}{81} - y^4}{\frac{2x}{3} + y}$$

Wolfram alpha confirms the answer from the answer sheet: [Wolframalpha answer](http://www.wolframalpha.com/input/?i=simplify%20%28%2816%28x%5E4%29%29/81-y%5E4%29/%28%28%282x%29/3%29%2by%29)

Answer: 
Multiplying by 81 on the top and the bottom gives

$\displaystyle\frac{16x^4-81y^4}{27(2x+3y)}=\frac{(4x^2-9y^2)(4x^2+9y^2)}{27(2x+3y)}=\frac{(2x-3y)(2x+3y)(4x^2+9y^2)}{27(2x+3y)}=$

$\frac{1}{27}(2x-3y)(4x^2+9y^2)$.
hint: try multiplying by $\frac{\frac{2x}{3}-y}{\frac{2x}{3}-y}$ and then $\frac{\frac{4x^2}{9}+y^2}{\frac{4x^2}{9}+y^2}$