Chapter 4 - Polynomials - 4.2 Negative Exponents and Scientific Notations - 4.2 Exercise Set - Page 245: 106

$6.51\times10^{3}$

Using the laws of exponents and the concepts of scientific notation, the given expression, $ (3.1\times10^7)(2.1\times10^{-4}) $, simplifies to \begin{array}{l} (3.1)(2.1)\times10^{7+(-4)} \\\\= 6.51\times10^{3} .\end{array}