help solving d^3 y / dx^3

Hi I can't seem to find the solution of the next question. Given is a parameter equasion: x = e^(-t) y=t^3. When i was solving this i got that: dx^3= e^-3t * (-3)
and d^3 y = 6. Then u got 6/ e^-3t * (-3) = -2e^3t
Now the solution in my book is : -6e^3t (1+3t+t^2) and it should be right...
Can anyone help me? ty