\(E=-\left(x^4+10x^2+9+6x^3+6x\right)+24\)
\(=-\left[\left(x^2+9\right)\left(x^2+1\right)+6x\left(x^2+1\right)\right]+24\)
\(=-\left(x^2+1\right)\left(x^2+9+6x\right)+24\)
\(=-\left(x^2+1\right)\left(x+3\right)^2+24\le24\)
\(E_{max}=24\) khi \(x=-3\)