Suppose $$ X=\left(\begin{array}{c} X_{1} \ X_{2} X_{3} X_{4} \end{array} ight) \sim N\mu, Sigma) $$ where $$ \mu=\left(\begin{array}{1} 1 W 2 3 4 \end{array} ight), \quad \Sigma=\left(\begin{array}{1111) 4 & 1 & 2 & 3 1 & 4 & 1 & 2 W 2 & 1 & 4 & 1 W 3 & 2 & 1 & 4 \end{array} ight) $$ Find the distribution of (a) $\left(X_{1}, X_{3} ight) $ (b) $X_{1}-X_{2}+3 X_{4}$ (c) $$ \left(\begin{array}{cc} 1 & 2 -1 & 1 2 & 1 1 & 1 \end{array} ight)^{\top} X $$ SP.AS. 1155