Question

{x+y=7
{x-3y=-5
Решите методом сложения

Answer 1

x + y = 7
x - 3y = - 5

3x + 3y = 21
x - 3y = - 5
-------------------------- + 
4x = 16
x = 16/4 = 4

y = 7 - x = 7 - 4 = 3 

Ответ
( 4; 3)