x^6y^9 : [x⁴y²z^5 : (-x²y⁴z³)]

x⁶y⁹ : [x⁴y²z⁵ : (-x²y⁴z³)]

= x⁶y⁹ : [(x⁴/-x²) (y²/y⁴) (z⁵/z³)]
= x⁶y⁹ : -x⁽⁴⁻²⁾ y⁽²⁻⁴⁾ z⁽⁵⁻³⁾
= x⁶y⁹ : -x²y⁻²z²
= -x⁽⁶⁻²⁾ y⁹ / y⁻²z²
= -x⁴ y⁹ . y² / z²
= -x⁴y¹¹ / z²