Derive expressions for the means and variances of the following linear combinations in terms of the means and covariances of the random variables X₁, X₂ , and X₃.
(a) X₁ - 2X₂
(b) -X₁ + 3X₂
(C) X₁ + X₂+x₃
(d) x₁+2x₂-x₃