To find the inverse function.

Question

I am trying to find the inverse function of the following equation $$f(x) = -x^2 {e^{-3x} \over 3} - 2x{e^{-3x} \over 9} - 2 {e^{-3x} \over 27} + {2 \over 27}$$ where $x \ge 0 $.
I know that the above equation is one-one and onto but I am not able to find the inverse. I have typed my workout below. I hope someone can help me. Thank you! $$f(x) = -9x^2 {e^{-3x} \over 27} - 6x{e^{-3x} \over 27} - 2 {e^{-3x} \over 27} + {2 \over 27}$$ $$(27 \times f(x)) = -9x^2 {e^{-3x} } - 6x{e^{-3x} } - 2 {e^{-3x} } + {2}$$ $$(27 \times f(x))- {2} = -9x^2 {e^{-3x} } - 6x{e^{-3x} } - 2 {e^{-3x} } $$ $$(27 \times f(x))- {2} = (-9x^2 - 6x - 2){e^{-3x} } $$