Find the general solution using the method of elimination

x'=x+2y+z

y'=6x-y

z'=-x-2y-z

I used a matrix and found

D^3 + D^2 - 12D=0

and then I think

x(t)= A + Be^(-4t) +Ce^(3t)

But then I don't know where to go, or how to solve it,

Please Help

Thanks