$In$ each $of$ Problems $11$ through $13,$ determine the values $ofr$ for which

the given differential equation has solutions $of$ the form $y=e^{rt}.$

$y^{\text{'}}+2y=0$

$y^{\text{'}\text{'}}+y^{\text{'}}-6y=0$

$y^{\text{'}\text{'}\text{'}}-3y^{\text{'}\text{'}}+2y^{\text{'}}=0$