Chapter 5 - Section 5.2 - More Work with Exponents and Scientific Notation - Exercise Set - Page 268: 73

$4.0\times10^{-7}$

Using laws of exponents, the given expression simplifies to \begin{align*} & \dfrac{3.6\times10^{-4}}{9\times10^2} \\\\&= (3.6\div9)(10^{-4}\div10^2) \\\\&= 0.4\times10^{-4-2} \\\\&= 0.4\times10^{-6} \\\\&= 4.0\times10^{-7} .\end{align*}