(a) 3x - 1y + 2z = 15
(b) 3x + 2y + 1z = 4
(c) 1x - 2y + 1z = 10

(1b + 1c: d) 4x + 2z = 14
(2a - 1c: e) 5x + 3z = 20

(4e - 5d) 2z = 10: z = 5
(3d - 2e) 2x = 2: x = 1

(-a) -3x + y - 2z = -15
      -3 + y - 10 = -15
                y = -15 + 10 + 3
                y = -2

(1, -2, 5)

(a) 3(1) - 1(-2) + 2(5) = 15: 3 + 2 + 10 = 15: 15 = 15
(b) 3(1) + 2(-2) + 1(5) = 4: 3 - 4 + 5 = 4: 4 = 4
(c) 1(1) - 2(-2) + 1(5) = 10: 1 + 4 + 5 = 10: 10 = 10