Question

x - y = 3
2x + y = 9

Answer 1

$+ \left \{ {{x - y = 3 } \atop 2x + y = 9}} \right. \\\\ 3x=12\\ x= \frac{12}{3} = 4$

Answer 2

x - y=3
2x + y=9
------------
3x = 12
x = 4
Substituindo...
x - y = 3
4 - y = 3
- y = - 1
y = 1

Espero Ajudar!