I can't solve a complex inequality

[analog fable]

Hello. I am a self-taught student and I have no one else to ask.
I read on the Internet that if an inequality is represented by terms that stand in different degrees, then on both sides of the roots of terms with even degrees the signs do not change, and with odd degrees they do. In the example given in the textbook, this worked.
I took an example from my head and solved the inequality using this method and it seems to me that I did not succeed
Such a rule does not exist, or did I do something wrong?

[sly plinth]

your solution is correct, except the dot x=8. x shouldn't be equal to 8

[fringe mauve]

(Same for -11)

[sly plinth]

he didn't paint -11

[analog fable]

Oh, yeah. The dot is my inattention. Thanks. I just gave neural networks this example, and they gave me a different answer, and special applications for solving inequalities refused to solve this
Thanks again

[fringe mauve]

If you want a Quick proof, any REAL number, to the power 2n, will alwyas be positive, that's why they "do not matter" (think of it as a square)

[valid fox]

!done

[south ridge]

If you are done with this channel, please mark your problem as solved by typing .close

[analog fable]

.close