6) (9x⁵y²/7z⁴)³ : (-9x⁴y¹⁰/7z³)⁴.

6) $$\left(\frac{9x^5y^2}{7z^4}\right)^3 : \left(-\frac{9x^4y^{10}}{7z^3}\right)^4 = \frac{(9x^5y^2)^3}{(7z^4)^3} : \frac{(9x^4y^{10})^4}{(7z^3)^4} = \frac{9^3x^{5\cdot 3}y^{2\cdot 3}}{7^3z^{4\cdot 3}} : \frac{9^4x^{4\cdot 4}y^{10\cdot 4}}{7^4z^{3\cdot 4}} = \frac{9^3x^{15}y^6}{7^3z^{12}} \cdot \frac{7^4z^{12}}{9^4x^{16}y^{40}} = \frac{9^3 \cdot 7^4 \cdot x^{15} \cdot y^6 \cdot z^{12}}{7^3 \cdot 9^4 \cdot x^{16} \cdot y^{40} \cdot z^{12}} = \frac{7}{9} \cdot \frac{x^{15-16}}{y^{40-6}} \cdot z^{12-12} = \frac{7}{9} \cdot \frac{1}{x} \cdot \frac{1}{y^{34}} = \frac{7}{9xy^{34}}$$

Ответ: 7/(9xy³⁴)