The side adjacent to an angle is one of the two sides that form the angle. $\displaystyle{\overline{{{B}{C}}}}\usepackage{cmbright}\Huge\$ does not form part of $\displaystyle\angle{A}\usepackage{cmbright}\Huge\$ , so it is the side opposite $\displaystyle\angle{A}\usepackage{cmbright}\Huge\$ .