[sahaete]

\[\begin{array}{l}
from\quad AX = B\quad Cramer's\;Law\quad {x_1} = \frac{{\det {A_1}}}{{\det A}}\\
\Rightarrow \quad \det A = {a_{11}}{C_{11}} + {a_{21}}{C_{21}} + {a_{31}}{C_{31}} + {a_{41}}{C_{41}}\\
{\kern 1pt} \quad \quad \quad \quad = 0 + 0 + 1 \cdot {C_{31}} + 2\left( { - 2} \right)\\
\quad \quad \;\; - 8 = {C_{31}} - 4\\
\Rightarrow \quad \\
\Rightarrow \quad \det {A_1} = \left| {\begin{array}{*{20}{c}}
0&{{a_1}}&{{a_2}}&{{a_3}}\\
0&{{b_1}}&{{b_2}}&{{B_3}}\\
2&{{c_1}}&{{c_2}}&{{c_3}}\\
1&{{d_1}}&{{d_2}}&{{d_3}}
\end{array}} \right| = {a_{11}}{C_{11}} + {a_{21}}{C_{21}} + {a_{31}}{C_{31}} + {a_{41}}{C_{41}}\\
\Rightarrow \quad \det {A_1} = 0 + 0 + 2\left( { - 4} \right) + 1\left( { - 2} \right) = - 10\\
then\quad {x_1} = \frac{{\det {A_1}}}{{\det A}} = \frac{{ - 10}}{{ - 8}} = \frac{5}{4} = 1.25
\end{array}\]