E$6\mathrm{.}24$ Consider the system represented in state variable form

x$($t$)=$Ax$($t$)+$Bu$($t$)$y$($t$)=$Cx$($t$)+$Du$($t$),$

where

A$=[010001$k$$k$$k$],$B$=[001]$C$=[100],$D$=[0].$

$($a$)$ What is the system transfer function? $($b$)$ For what values of k is the system stable?