1) $$\frac{x^2 - 9y^2}{16x^2 - 9y^2} : \frac{x^2 + 6xy + 9y^2}{16x^2 - 24xy + 9y^2} = \frac{(x-3y)(x+3y)}{(4x-3y)(4x+3y)} : \frac{(x+3y)^2}{(4x-3y)^2} = \frac{(x-3y)(x+3y)}{(4x-3y)(4x+3y)} \cdot \frac{(4x-3y)^2}{(x+3y)^2} = \frac{(x-3y)(4x-3y)}{(4x+3y)(x+3y)}$$
Ответ: $$\frac{(x-3y)(4x-3y)}{(4x+3y)(x+3y)}$$