Resposta:

[tex]a) \: x - y = 1 \\ - x + 2y = 3 \\ \\ x = 1 + y \\ - x + 2y = 3 \\ \\ - (1 + y) + 2y = 3 \\ - 1 - y + 2y = 3 \\ y = 3 + 1 \\ y = 4 \\ \\ x = 1 + y \\ x = 1 + 4 \\ x = 5 \\ \\ (x.y) = (5.4)[/tex]

[tex]b) \: x = 3y \\ x - 2y = 3 \\ \\ 3y - 2y = 3 \\ y = 3 \\ \\ x = 3y \\ x = 3 \times 3 \\ x = 9 \\ \\ (x.y) = (9.3)[/tex]

[tex]c) \: 2x + y = - 3 \\ x + y = - 1 \\ \\ 2x + y = - 3 \\ x = - 1 - y \\ \\ 2 \times ( - 1 - y) + y = - 3 \\ - 2 - 2y + y = - 3 \\ - y = - 3 + 2 \\ - y = - 1 \times ( - 1) \\ y = 1 \\ \\ x = - 1 - 1 \\ x = - 2[/tex]