A Domain of Asymptotic Stability

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For a system $$x'=-x^4+2x^5+y$$ $$y'=-x^5-y.$$ I found the Liapunov function as $$V(x,y)=(x+y)^2+2((x^5)/5-(x^6)/6)$$. A domain of asymptotic stability will be the interior of the largest level curve
which is within $$-1<x<1$$ Now, I cannot proceed since I couldn't find the simple closed curve by using this Liapunov function. Any helps will be appreciated.