4. Consider the ARMA(1,2) process Xt +axt-1 = Zt + BZt-2. (a) (10 points) Under which conditions does the process Xt becomes stationary and invert- ible? (b) (10 points) Given the observations X1, ..., Xn, what is your best prediction for Xn+1 and Xn+2 at the forecast origin Xn? What is the corresponding forecasting error