Time equals distance divided by rate: $\displaystyle\frac{{{4.5}\times{{10}}^{{12}}}}{{{3}\times{{10}}^{{8}}}}\usepackage{cmbright}\Huge\$

Regroup and divide the numbers: $\displaystyle{\left(\frac{{4.5}}{{3}}\right)}{\left(\frac{{{10}}^{{12}}}{{{10}}^{{8}}}\right)}={\left({1.5}\right)}{\left(\frac{{{10}}^{{12}}}{{{10}}^{{8}}}\right)}\usepackage{cmbright}\Huge\$

Use the quotient rule: $\displaystyle{1.5}\times{{10}}^{{{12}-{8}}}={1.5}\times{{10}}^{{4}}\usepackage{cmbright}\Huge\$