$\left\{\begin{matrix}x=3\left(1-3y_{1}\right)\text{, }y=-\frac{7-36y_{1}}{1-4a}\text{, }z=\frac{-36ay_{1}+18y_{1}-3a-1}{1-4a}\text{, }&a\neq \frac{1}{4}\\x=\frac{5}{4}=1.25\text{, }y\in \mathrm{R}\text{, }z=\frac{y+10}{4}\text{, }&y_{1}=\frac{7}{36}\text{ and }a=\frac{1}{4}\end{matrix}\right.$