It appears that you set up the proportion incorrectly. $\displaystyle{\overline{{{L}{A}}}}\usepackage{cmbright}\Huge\$ corresponds to $\displaystyle{\overline{{{F}{I}}}}\usepackage{cmbright}\Huge\$ , and $\displaystyle{\overline{{{L}{K}}}}\usepackage{cmbright}\Huge\$ corresponds to $\displaystyle{\overline{{{F}{M}}}}\usepackage{cmbright}\Huge\$ . Now set up the correct proportion and solve.