`a)`

`2(x+3)  - x^2 - 3 =0`

`=> 2x+6-x^2-3=0`

`=>-x^2+2x+3=0`

`=> x^2 - 2x - 3 =0`

`=> x^2 - 3x + x - 3 = 0`

`=> x (x-3) + (x-3) =0`

`=> (x-3)(x+1)=0`

`=>x-3=0` hoặc `x+1=0`

`+)x-3=0=>x=3`

`+)x+1=0=>x=-1`

Vậy `x \in {3;-1}`

`b)`

`(3x-1)^2 - 25 =0`

`=> (3x-1)^2 - 5^2 =0`

`=> (3x-1-5)(3x-1+5)=0`

`=>(3x-6)(3x+4)=0`

`=>3x-6=0` hoặc `3x+4=0`

`+)3x-6=0=>3x=6=>x=2`

`+)3x+4=0=>3x=-4=>x=-4/3`

Vậy `x \ in {2;-4/3}`

`c)`

`(2x^2 - 7x) - 4x+14=0`

`=> x (2x-7) - 2 (2x-7) =0`

`=> (x-2)(2x-7)=0`

`=>x-2=0` hoặc `2x-7=0`

`+)x-2=0=>x=2`

`+)2x-7=0=>2x=7=>x=7/2`

Vậy `x \in {2;7/2}`

 

$ c) 2 ( x + 3 ) -x^2 - 3 =0 $

$ ⇒ 2x + 6 - x^2 - 3 =0 $

$ ⇒ - x^2 + 2x + 3 =0 $

$ ⇒ x^2 - 2x - 3 =0 $

$ ⇒ ( x - 3 ) ( x +1 )=0$

$ ⇒ $\(\left[ \begin{array}{l}x-3=0\\x+1=0\end{array} \right.\) 

$ ⇒ $ \(\left[ \begin{array}{l}x=3\\x=-1\end{array} \right.\) 

Vậy $ x = 3 $ hoặc $ x =-1 $