`#Sad`
`a)`
`4x(7x-5)-7x(4x-2) = -12`
`⇔ 28x^2-20x-28x^2+14x = -12`
`⇔ -6x = -12`
`⇔ x = 2`
`\text{Vậy S=}` `{2}`
`b)`
`(x+3)(x-2)+3x = 4(x+3/4)`
`⇔ x^2-2x+3x-6+3x = 4x+3`
`⇔ x^2+4x-6-4x-3 = 0`
`⇔ x^2-9 = 0`
`⇔ x^2 = 9`
`⇔` \(\left[ \begin{array}{l}x=3\\x=-3\end{array} \right.\)
`\text{Vậy S=}` `{3; -3}`
