Answer
$6.51\times10^{3}$
Work Step by Step
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}