264. System of four linear equations. Solve the following system of equations:

$$
\left\{\begin{aligned}
x+7 y+3 v+5 u & =16 \\
8 x+4 y+6 v+2 u & =-16 \\
2 x+6 y+4 v+8 u & =16 \\
5 x+3 y+7 v+u & =-16
\end{aligned}\right.
$$
264. If we interchange $x$ and $u$, $y$ and $v$ in equation (1), we will get equation (4), but with the opposite sign on the right-hand side. If we do the same in equation (2), we will get equation (3) with the same caveat. Therefore, $u=-x$ and $v=-y$. Substituting these equalities into (1) and (2), we get

$$
\begin{aligned}
-4 x+4 y & =16 \\
6 x-2 y & =-16 \\
\hline 8 x \quad & =-16
\end{aligned}
$$

Thus ${ }^{\star}, x=-2, y=2, v=-2, u=2$.