$6-$ If $x1,x2,$ and $x3$ have the means $4,9,$ and $3$ the variances $3,7,$ and $5,$ and cov$(x_1,x_2)=1,$ cov$(x_1,x_3)=-2,$cov$(x_2,x_3)=-3,$ find the mean and the variance of

a$)Y=2x_1-3x_2+4x_3$;

b$)Z=x_1+2x_2-3x_3.$

c$)$ Find cov$(Y,Z).$