$8a-5\overset{11}{\mathop{\equiv }}\,1-4a\Rightarrow 12a\overset{11}{\mathop{\equiv }}\,6\xrightarrow[(6,11)=1]{\div 6}2a\overset{11}{\mathop{\equiv }}\,1\overset{11}{\mathop{\equiv }}\,12\xrightarrow[(11,2)=1]{\div 2}$ $a\overset{11}{\mathop{\equiv }}\,6\Rightarrow \left\{ \begin{matrix} {{a}^{2}}\overset{11}{\mathop{\equiv }}\,36  \\ 3a\overset{11}{\mathop{\equiv }}\,18  \\ \end{matrix} \right.\Rightarrow {{a}^{2}}-3a\overset{11}{\mathop{\equiv }}\,18\Rightarrow {{a}^{2}}-3a+2\overset{11}{\mathop{\equiv }}\,20\overset{11}{\mathop{\equiv }}\,9$