Chapter 4 - Polynomials - 4.2 Negative Exponents and Scientific Notations - 4.2 Exercise Set - Page 244: 76

$\dfrac{z^{4}}{xy^{6}}$

Using the laws of exponents, the given expression, $ (x^3y^{-4}z^{-5})(x^{-4}y^{-2}z^9) $, is equivalent to \begin{array}{l}\require{cancel} x^{3+(-4)}y^{-4+(-2)}z^{-5+9} \\\\= x^{-1}y^{-6}z^{4} \\\\= \dfrac{z^{4}}{x^{1}y^{6}} \\\\= \dfrac{z^{4}}{xy^{6}} .\end{array}