\(x^4-8x+63=\left(x^2+ax+b\right)\left(x^2+cx+d\right)\)
\(=x^4+\left(a+c\right)x^3+\left(b+d+ac\right)x^2+\left(ad+bc\right)x+bd\)
\(\Rightarrow\left\{{}\begin{matrix}a+c=0\\b+d+ac=0\\ad+bc=-8\\bd=63\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}a=-4\\b=7\\c=4\\d=9\end{matrix}\right.\)
\(\Rightarrow x^4-8x+63=\left(x^2-4x+7\right)\left(x^2+4x+9\right)\)