A fair die is rolled 2 times.

A fair die is rolled 2 times. Let X1 and X2 denote the number of points showing on the first and second rolls, respectively. Let U=X1+X2 and V=X1-X2.
(a)Find the mean and variance of V.
(b)Find the covariance of U and V.
(c)Show that U and V are not independent