That really depends on what parts of the parallelogram 'x' and 'y' are. Are they a pair of opposite sides ? A pair of adjacent sides ? One side and one diagonal ? A diagonal and an angle ? A side and...
For a discrete probability distribution, you add up x*P(x) over all possible values of x, where P(x) is the probability that the random variable X takes the value x.For a continuous distribution you...