You may have substituted the 4 in for R instead of for the A. The initial amount $\displaystyle{A}={4}\usepackage{cmbright}\Huge\$ , the time $\displaystyle{t}={9}\usepackage{cmbright}\Huge\$ , and the half-life $\displaystyle{h}={3}\usepackage{cmbright}\Huge\$ . Substitute into the formula to get $\displaystyle{R}={4}{{\left(\frac{{1}}{{2}}\right)}}^{{\frac{{9}}{{3}}}}\usepackage{cmbright}\Huge\$ , and then simplify.