If a prescription drug's half-life is 3 days and a patient currently has 4 milligrams of the drug in his system, how many milligrams of the drug will be in his body after 9 days?

Use the formula: $\displaystyle{R}={A}{{\left(\frac{{1}}{{2}}\right)}}^{{\frac{{t}}{{h}}}}\usepackage{cmbright}\Huge\$