1. Questão

[tex]\begin{cases} 2x + y = 1 \\ x - 3y = 4\end{cases}[/tex]

[tex]\begin{cases} y = 1 - 2x \\ x - 3y = 4\end{cases}[/tex]

[tex]x - 3(1 - 2x) = 4[/tex]

[tex] = 1[/tex]

[tex]y = 1 - 2 \times 1[/tex]

[tex]y = - 1[/tex]

[tex](x,y) = (1, - 1)[/tex]

[tex]\begin{cases} 2 \times 1 + ( - 1) = 1 \\ 1 - 3 \times ( - 1) = 4\end{cases}[/tex]

[tex]\begin{cases} 1 = 1 \\ 4 = 4\end{cases}[/tex]

[tex]\boxed{\boxed{\color{cyan} {  (x,y) = (1, - 1)}}} \: \color{darkturquoise}ou \: seja \: letra \: \copyright \: ✓[/tex]

2. Questão

[tex]\begin{cases}3x - 4y = 1 \\ x - 2y = 3 \end{cases}[/tex]

[tex]\begin{cases}3x - 4y = 1 \\ x = 3 + 2y \end{cases}[/tex]

[tex]3 \: (3 + 2y) - 4y = 1[/tex]

[tex]y = - 4[/tex]

[tex]x = 3 + 2 \times ( - 4)[/tex]

[tex]x = - 5[/tex]

[tex](x,y) = ( - 5, - 4)[/tex]

[tex]\begin{cases}3 \times ( - 5) - 4 \times ( - 4) = 1 \\ - 5 - 2 \times ( - 4) = 3 \end{cases}[/tex]

[tex]\begin{cases} 1 = 1 \\ 3 = 3\end{cases}[/tex]

[tex]\boxed{\boxed{\color{cyan} {  (x,y) = ( - 5, - 4)}}} \: \color{darkturquoise}ou \: seja \: letra \: (a) \: ✓[/tex]

3. Questão

[tex]\begin{cases}5x + 3y = 3 \\ 4x - y = - 18 \end{cases}[/tex]

[tex]\begin{cases}5x + 3y = 3 \\ y = 18 + 4x \end{cases}[/tex]

[tex]5x + 3(18 + 4x = 3[/tex]

[tex]x = - 3[/tex]

[tex]y = 18 + 4 \times ( - 3)[/tex]

[tex]y = 6[/tex]

[tex](x,y) = ( - 3,6)[/tex]

[tex]\begin{cases}5x( - 3) + 3 \times 6 = 3 \\ 4x( - 3) - 6 = - 18 \end{cases}[/tex]

[tex]\begin{cases}3 = 3 \\ - 18 = - 18 \end{cases}[/tex]

[tex]\boxed{\boxed{\color{cyan} {  (x,y) = ( - 3, 6)}}} \: \color{darkturquoise}ou \: seja \: letra \: (b) \: ✓[/tex]

Espero ter colaborado, bons estudos ❤.