 $$\Bbb F : \ \ \cdots \to R^0 \overset{ \begin{pmatrix} -b & -c\\ a & b \end{pmatrix}}{\longrightarrow} R^0 \overset{ \begin{pmatrix} -b & -c\\ a & b \end{pmatrix}}{\longrightarrow}R^0 \overset{\begin{pmatrix} a & b \end{pmatrix}}{\longrightarrow} I \to 0