Solve the following system of equations using Gaussian elimination: (5d - 20e - 10f = -85), (-2d - e + 39f = 223), and (4d - 13e - 19f = -125).
(a) (d = 7, e = 5, f = -3)
(b) (d = -3, e = 2, f = -5)
(c) (d = 5, e = -3, f = 7)
(d) (d = 2, e = -5, f = 3)