\(a,3x^4y^5z^2⋮5x^ny^nz^2\Leftrightarrow\left\{{}\begin{matrix}n\le4\\n\le5\end{matrix}\right.\Leftrightarrow n\le4\\ b,4x^ny^3⋮3x^2y^n\Leftrightarrow\left\{{}\begin{matrix}n\ge2\\n\le3\end{matrix}\right.\Leftrightarrow2\le n\le3\\ \Leftrightarrow n\in\left\{2;3\right\}\)