a)
`(x-2)(x+4)=0`
⇔\(\left[ \begin{array}{l}x-2=0\\x+4=0\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x=2\\x=-4\end{array} \right.\)
b)
`(x-2)(x+15)=0`
⇔\(\left[ \begin{array}{l}x-2=0\\x+15=0\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x=2\\x=-15\end{array} \right.\)
c)
`(7-x)(x+19)=0`
⇔\(\left[ \begin{array}{l}7-x=0\\x+19=0\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x=7\\x=-19\end{array} \right.\)
