\(\left(x+1\right)\left(x-2\right)^2+x^2\left(4-x\right)=13\)
\(\Leftrightarrow\left(x+1\right)\left(x^2-4x+4\right)+x^2\left(4-x\right)-13=0\)
\(\Leftrightarrow x^3-4x^2+4x+x^2-4x+4+4x^2-x^3-13=0\)
\(x^2-9=0\Leftrightarrow\left(x-3\right)\left(x+3\right)=0\)
\(\Leftrightarrow\left[\begin{array}{nghiempt}x=3\\x=-3\end{array}\right.\)