`2x(x-3)-5(3-x) = 0`
`⇔ 2x^2 - 6x - 5(3-x) = 0`
`⇔ 2x^2 - 6x - 15 + 5x = 0`
`⇔ 2x^2 - x - 15 = 0`
`⇔ (2x^2 + 5x) + (-6x-15) = 0`
`⇔ x(2x + 5) - 3(2x+5) = 0`
`⇔ (2x+5)(x-3) = 0`
`⇔`\(\left[ \begin{array}{l}2x+5=0\\x-3=0\end{array} \right.\)
`⇔`\(\left[ \begin{array}{l}x=-\dfrac{5}{2}\\x=3\end{array} \right.\)
Vậy `S = {-5/2 , 3}`
