4. Suppose that the MGF of a random variable X is of the form M(t) = 1/8e^{2t} + 3/8e^{-2t} + 1/2e^{t}, -infinity < t < infinity. (a) Using the above MGF, derive the mean of X. (b) Using the uniqueness of the MGF, derive the probability distribution of X. Hint: Think of a discrete random variable that would have this particular MGF. Then, derive P(X < 1.5).