`a, 25.x-36.x-14.x=500`
`⇒ (25-36-14).x=500`
`⇒ -25.x=500`
`⇒ x = 500 : (-25)`
`⇒ x = -20
`b, x.(x²-9)=0`
\(⇒\left[ \begin{array}{l}x=0\\x^{2}-9=0\end{array} \right.\)
\(⇒\left[ \begin{array}{l}x=0\\x^{2}=9\end{array} \right.\)
\(⇒\left[ \begin{array}{l}x=0\\x=±3\end{array} \right.\)
$\text{Vậy x ∈ {0; ±3}}$