a. $(x-3)$$.$$(x+5)$ $\geq$ $0$
⇔ \(\left[ \begin{array}{l}x-3\geq0\\x+5\geq0\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x\geq3\\x-5\end{array} \right.\)
b. $x(x-4)$ $<$ $0$
$Th1$:\(\left[ \begin{array}{l}x>0\\x$\geq$ -4<0⇔x<4\end{array} \right.\)
$Th2$:\(\left[ \begin{array}{l}x<0\\x-4>0<=> x>4\end{array} \right.\)