Chapter 4 - Polynomials - 4.2 Negative Exponents and Scientific Notations - 4.2 Exercise Set - Page 245: 115

$3.0\times10^{-21}$

Using the laws of exponents and the concepts of scientific notation, the given expression, $ \dfrac{7.5\times10^{-9}}{2.5\times10^{12}} $, simplifies to \begin{array}{l} (7.5\div2.5)\times10^{-9-12} \\\\= 3.0\times10^{-21} .\end{array}