Check the following for stability

$($a$)u(k)=0\mathrm{.}5u(k-1)-0\mathrm{.}3u(k-2)$; $($Do it manually$)$

$($b$)u(k)=1\mathrm{.}6u(k-1)-u(k-2)-0\mathrm{.}5u(k-3)$; $($Using computer, Hint: using the Matlab

command: roots$)$