probability question pls help

Q4 (a) Let X1, ... , X, be independent exponential random variables with parameter a. Compute the moment generating function of Y = X1+ . . + Xn, and hence show that Y = y(n, a).(b) Let X be a standard normal random variable. Show that the moment generating function of X2 is Mxz(t) = (1 - 2t)-1/2, t