$a)$

$\{\begin{array}{l}x+3\ne 0\\ x^2\ne 0\end{array}\Rightarrow \{\begin{array}{l}x\ne -3\\ x\ne 0\end{array}\Rightarrow D=\mathbb{R}\backslash \left\{-3,0\right\}$

$\frac{6x}{\sout{x+3}}\cdot \frac{\sout{x+3}}{x^2}=\frac{6x}{x^2}=\frac{6}{x}$