sửa "5a^2=5ax^2"
: \(f\left(x\right)=ax^3+5ax^2+bx+4\)
\(f\left(-1\right)=0\Rightarrow-a+5a-b+4=0\Rightarrow4a-b+4=0\)
\(\Rightarrow\left\{{}\begin{matrix}a-b=3\\3a+\left(a-b\right)+4=0\end{matrix}\right.\)\(\Rightarrow\left\{{}\begin{matrix}a=\dfrac{-7}{3}\\b=\dfrac{-16}{3}\end{matrix}\right.\)
\(f\left(x\right)=\dfrac{-7}{3}x^3-\dfrac{35}{3}x^2-\dfrac{16}{3}x+4\)