Answer
$4.2 \times 10^{3}$
Work Step by Step
$\dfrac{a^m}{a^n}=a^{m-n}, a\ne0$
Divide the corresponding parts while using the rule above to obtain:
$=\dfrac{8.4}{2} \times \dfrac{10^{-5}}{10^{-8}}
\\=4.2 \times 10^{-5-(-8)}
\\=4.2 \times 10^{-5+8}
\\=4.2 \times 10^{3}$
