The distance between the cities is $\displaystyle\frac{{{921},{000}\ \text{ meters}}}{{1}}\cdot\frac{{{1}\ \text{ kilometer}}}{{{1},{000}\ \text{ meters}}}\usepackage{cmbright}\Huge\$ , or 921 kilometers.

$\displaystyle\frac{{{921}\ \text{ km}}}{{n}}=\frac{{{307}\ \text{ km}}}{{{1}\ \text{ hour}}}\usepackage{cmbright}\Huge\$

$\displaystyle{n}=\frac{{921}}{{307}}={3}\usepackage{cmbright}\Huge\$