$4$ Solve the following second order equations with constant coefficients.

a$)y^{\text{'}\text{'}}-6y^{\text{'}}+9y=0$

b$)2y^{\text{'}\text{'}}+y^{\text{'}}+y=0$;$y(0)=1$;$y^{\text{'}}(0)=-2$

c$)y^{\text{'}\text{'}}=0$

$1$