Substitute 3.6 for R in the formula: $\displaystyle{3.6}={\log{{\left(\frac{{A}}{{A}_{{0}}}\right)}}}\usepackage{cmbright}\Huge\$

Rewrite as an exponential equation: $\displaystyle{{10}}^{{3.6}}=\frac{{A}}{{A}_{{0}}}\usepackage{cmbright}\Huge\$

Use a calculator and multiply both sides by $\displaystyle{A}_{{0}}\usepackage{cmbright}\Huge\$ : $\displaystyle{A}\approx{3},{981}{A}_{{0}}\usepackage{cmbright}\Huge\$