[tex]\displaystyle\\
\begin{cases}
x+y=351\\
x-y = 7\cdot \dfrac{y}{3} ~~~\Big|\cdot 3
\end{cases}\\\\\\
\begin{cases}
x+y=351\\
3x-3y = 7y
\end{cases}\\\\\\
\begin{cases}
x+y=351~~~~~\Big|\cdot 10\\
3x-10y = 0
\end{cases}\\\\\\
\begin{cases}
10x+10y=3510\\
3x-10y = 0
\end{cases}\\
---------- \text{Adunam ecuatiile}\\
~~13x~~~~~/~~=3510\\\\
x = \frac{3510}{13}=\boxed{270} \\\\
x+y=351~~\Longrightarrow~~y = 351-x=351-270=\boxed{81}[/tex]