6. (5 points) We have the following information regarding the random variables X1, X2 and X3: Var(X1) = Var(X2) = Var(X3), PX1,X2 = 0.2, PX1,X3 = 0.5, PX2, X3 = 0.9. Calculate the correlation between X1 + X3 and X2 + X3. (i.e. calculate PX,+X3,X2+X3. )