Деление с учетом знака
x^2(-x^2-64)<=64(-x^2-64)

x²(-x²-64)≤64(-x²-64)
x²≥64
/x/≥8
x∈(-∞,-8/∪ x∈/8,∞)
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-x^2(x^2+64)<= -64(x^2+64)
x^2>=64
x1=8
x2= -8
(-&; -8] [8; +&)