You may have thought that because 10 is 4 more than 6, AH would be 4 more than 4. However, in similar triangles, side lengths are proportional. $\displaystyle{\overline{{{B}{L}}}}\usepackage{cmbright}\Huge\$ corresponds to $\displaystyle{\overline{{{A}{H}}}}\usepackage{cmbright}\Huge\$ , and $\displaystyle{\overline{{{B}{T}}}}\usepackage{cmbright}\Huge\$ corresponds to $\displaystyle{\overline{{{A}{M}}}}\usepackage{cmbright}\Huge\$ . Now set up the correct proportion and solve.