$6\mathrm{.}2$ Write the H$($z$)$ z$-$domain transfer function equations for the filters described by the following difference equations:

$($a$)$ y$($n$)=$ x$($n$)$ y$($n$2),$

$($b$)$ y$($n$)=$ x$($n$)+3$x$($n$1)+2$x$($n$2)$ y$($n$3),$

$($c$)$ y$($n$)=$ x$($n$)+$ x$($n$1)+$ x$($n$3)+$ x$($n$4)$ y$($n$2).$