Covariance matrix of 2-dimensional distributions

Let (X, Y) be a point uniformly distributed in the triangle with vertices (-1, 0), (0, 0) and (0, 1) in the plane.
a) Find the covariance matrix of the (X, Y) two dimensional distribution.
b) Let be Z= X+2Y.Find the covariance matrix of the (X, Z) two dimensional distribution.

Thank you very much in advance!