Z00049. Solve the following systems of linear equations using Gaussian Elimination and back substitution. Check your answers by substituting them into the original system.

(1.1.1) 6x₁ – 3x₂ – 6x₃ = – 63 ; 2x₁ – 3x₂ + 5x₃ = 17 ; – x₁ – 2x₂ – x₃ = 15

(1.1.2) – 5x₁ – 4x₂ – 4x₃ = 33 ; 3x₁ + x₂ = – 25 ; – 4x₂ + x₃ = – 7