Question

x+ y - 2z =13
y + z = 9
-z -2 =5

Answer 1

y + z = 9
;;;;- z - 2 = 5    (somando o sistema)
_______
y - 2 = 14
y = 14 + 2
y = 16.

---- > y + z = 9
16 + z = 9
16 - 9 = z
7 = z

-----> x + y - 2z = 13
x + 16 - 2(7) = 13
x + 16 - 14 = 13
x = 13 - 16 + 14
x = 27 - 16
x = 11

ou seja: x = 11 ; y = 16 ; z = 7