hi
i need little help
$\displaystyle \left\{\begin{matrix}
x+z+t=0\\
ax+y+(a-1)z+t=0\\
x-y+2z+at=0\\
2x+ay+z+2t=0
\end{matrix}\right.$
thanks.
To do what? Solve the system of equations? Find the value of a such that these equations have a non-trival solution? That will happen if and only if the determinant is 0.