Doubt while solving a third order differential equation

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I solved $$y(x) = -3e^{-3x}+3\cos2x - 2\sin2x$$ And reached $$y^{(3)}+3y''+4y'+12y=0$$

Now it's graph looks periodic on the right side of the $x$-axis . I was wondering under what conditions is $$y(x)\approx 3\cos2x-2\sin2x$$