criterion

$($c$)$ For the characteristic equation $1+\frac{K_c}{(S+1)(\frac{S}{2}+1)(\frac{S}{3}+1)}=0,($a$)$ find the value of Kc for which the system is stable. $($b$)$ For the value of Kc for which the system is on the threshold of instability, determine the roots of the characteristic equation