`a)3x(x+1)-(x-2)(3x+1)=12`
`⇔3x²+3x-(3x²+x-6x-2)=12`
`⇔3x²+3x-3x²-x+6x+2=12`
`⇔8x+2=12`
`⇔8x=12-2`
`⇔8x=10`
`⇔x=10/8`
`⇔x=5/4`
Vậy `x=5/4`
`b)(3x²-x+1)(x-1)+x²(4-3x)=5/2`
`⇔3x³-3x²-x²+x+x-1+4x²-3x³=5/2`
`⇔2x-1=5/2`
`⇔2x=5/2+1`
`⇔2x=5/2+2/2`
`⇔2x=7/2`
`⇔x=7/2:2`
`⇔x=7/2 . 1/2`
`⇔x=7/4`
Vậy `x=7/4`
`c)(x+3)(5x-1)=5(x+1)(x-2)`
`⇔(x+3)(5x-1)-5(x+1)(x-2)=0`
`⇔5x²-x+15x-3+(-5x-5)(x-2)=0`
`⇔5x²-x+15x-3-5x²+10x-5x+10=0`
`⇔19x+7=0`
`⇔19x=-7`
`⇔x=-7/19`
Vậy `x=-7/19`
`d)(2x-3)(x+1)=2x(x-1)`
`⇔(2x-3)(x+1)-2x(x-1)=0`
`⇔2x²+2x-3x-3-2x²+2x=0`
`⇔x-3=0`
`⇔x=3`
Vậy `x=3`
