Giải hệ phương trình sau?

a.$\left\{\begin{array}{l}\frac{1}{x-2}+\frac{1}{y-1}=2\\ \frac{2}{x-2}-\frac{3}{y-1}=1\end{array}\right.$                    b,$\left\{\begin{array}{l}\frac{3}{2x-1}-\frac{6}{3-y}=-1\\ \frac{1}{2x-1}-\frac{1}{3-y}=0\end{array}\right.$