Answer
$2.1\times10^{3}$
Work Step by Step
$$\frac{8.4\times10^{8}}{4\times10^{5}}$$
Regroup like terms then simplify
$$=(\frac{8.4}{4})(\frac{10^{8}}{10^{5}})$$
When dividing exponential expressions with the same non-zero base, subtract the exponent of the denominator from the exponent of the numerator.
$$=2.1\times10^{(8-5)}$$
$$=2.1\times10^{3}$$
