Chapter 4 - Section 4.1 - Integer Exponents and Scientific Notation - 4.1 Exercises - Page 278: 126

$\dfrac{b^{10}}{a^{12}}$

Using the laws of exponents, the given expression, $ \dfrac{a^{-9}b^6}{a^3b^{-4}} ,$ is equivalent to \begin{array}{l}\require{cancel} a^{-9-3}b^{6-(-4)} \\\\= a^{-9-3}b^{6+4} \\\\= a^{-12}b^{10} \\\\= \dfrac{b^{10}}{a^{12}} .\end{array}