Question

resolva o seguinte sistema

[2x-y=7
x+3y=-7

Answer 1

$\begin{cases}2x-y=7\\ x+3y=-7\end{cases}\\\\\\\\ x+3y=-7\to~~ x=-3y-7\\\\\\ 2x-y=7\to~~ 2(-3y-7)-y=7\to~~ -6y-14-y=7\to\\\\ -6y-y=7+14\to~~ -7y=21~(-1)\to~~ 7y=-21\to~~y=- \dfrac{21}{7} \to\\\\ \boxed{y=-3}\\\\\\ x=-3y-7\to~~ x=-3.(-3)-7\to~~ x=9-7\to~~ \boxed{x=2}\\\\\\\\ \large\boxed{\boxed{S=\{2~;~-3\}}}$

Answer 2

Método da Adição:

2x - y = 7      (3)
x + 3y = - 7  

6x - 3y = 21
x + 3y = - 7 (+)
7x = 14
x = 14/7
x = 2

2x - y = 7
2.2 - y = 7
4 - 7 = y
- 3 = y
y = - 3