$18\mathrm{.}7$ Derive expressions for the one$-,$ two$-,$ and three$-$period forecasts, hat$(y{)}_T(1),$hat$(y{)}_T(2),$ and hat$(y{)}_T(3),$ for the second$-$order moving average process MA$(2).$ What are the variances of the errors for these forecasts? What is the variance of the error for the $l-$period forecast, with $l>3?$