Ola Alice

2x + 5y - z = 15 (adicionei o y)

3x - 2y + 3z = 12

x - 5y + 9z = 23

z = 2x + 5y - 15

3z = 6x + 15y - 45

9z = 18x + 45y - 135

3x - 2y + 6x + 15y - 45 = 12

9x + 13y = 57

x - 5y + 18x + 45y - 135 = 23

19x + 40y = 158

9x + 13y = 57

19x + 40y = 158

360x + 520y = 2280

247x + 520y = 2054

113x = 226

x = 2

18 + 13y = 57

13y = 39

y = 3

z = 2x + 5y - 15

z = 2*2 + 5*3 - 15

z = 4

x + y + z = 2 + 3 + 4 = 9 (A)

pronto