4x + 5y + 2z=-11
-2x + 2y – 2z = 10
5x – y + 2z = 13

(x , y , z) = (6 , 5 , -6)

Step-by-step explanation:

We can not deal with three equations at once. So the overall thinking process:

In other words...

(1) -4x + 5y + 2z=-11

(2) -2x + 2y – 2z = 10

(3) 5x – y + 2z = 13

(1) + (2) =>

     -4x -2x + 5y + 2y + 2z - 2z = -11 + 10 => -6x + 7y = -1 (4)

(2) + (3) =>

     -2x + 5x + 2y - y - 2z + 2z = 10 + 13 => 3x + y = 23 => 6x + 2y = 46 (5)

(4) + (5) => -6x + 6x + 7y + 2y = -1 + 46 => 9y = 45 => y = 5

(y = 5)  (5) => 6x + 10 = 46 => x = 6

(x = 6 , y = 5)  (3) => 5 * 6 - 5 + 2z = 13 => 30 - 5 + 2z = 13 => 2z = 13 - 25

          => 2z = -12 => z = -6

a. is correct