\(a,ĐK:-5x\ge-5\Leftrightarrow x\le1\\ b,ĐK:\left\{{}\begin{matrix}x+1\ne0\\\dfrac{2}{-x+3}\ge0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x\ne-1\\3-x>0\left(3-x\ne0;2>0\right)\end{matrix}\right.\\ \Leftrightarrow\left\{{}\begin{matrix}x\ne1\\x< 3\end{matrix}\right.\)