a) \(\left(x+a\right)\left(x^2+bx+16\right)\)
\(=x\left(x^2+bx+16\right)+a\left(x^2+bx+16\right)\)
\(=x^3+bx^2+16x+ax^2+abx+16a\)
\(=x^3+\left(a+b\right)x^2+\left(16+ab\right)x+16a\)
b) Ta có: \(\hept{\begin{cases}M=x^3+\left(a+b\right)x^2+\left(16+ab\right)x+16a\\N=x^3-64\end{cases}}\)
Cân bằng hệ số: \(\hept{\begin{cases}a+b=0\\16+ab=0\\16a=-64\end{cases}}\Leftrightarrow\hept{\begin{cases}a=-4\\4\end{cases}}\)