You have the (X) in a single substitute the equation of the second equation. Then, in an expression have the same (Y). Is a linear equation that that is easily solved. After finding the back of the basis of the expression at the beginning.
[tex]\begin{cases}x=-4y+4\\3x-7y=-7\end{cases}[/tex]
[tex]\begin{cases}x=-4y+4\\3(-4y+4)-7y=-7\end{cases}[/tex]
[tex]\begin{cases}x=-4y+4\\-12y+12-7y=-7\end{cases}[/tex]
[tex]\begin{cases}x=-4y+4\\-19y=-19|:(-19)\end{cases}[/tex]
[tex]\begin{cases}x=-4*1+4\\y=1\end{cases}[/tex]
[tex]\begin{cases}x=0\\y=1\end{cases}[/tex]
