[tex]\begin{gathered} -x+3y=20\ldots\ldots\ldots\ldots...(1)_{} \\ 7y=x\ldots\ldots.\ldots\ldots\ldots\ldots.....(2) \\ -7y+3y=20 \\ -4y=20 \\ \text{divide both sides by -4} \\ y=\frac{20}{-4} \\ y=-5 \\ 7y=x \\ 7\times-5=x \\ x=-35 \end{gathered}[/tex]
