`a)`

`4( x+ 1) ^2+ ( 2x-1) ^2 -8 ( x+ 1)( x-1) =11`

`4x^2+8x+4+4x^2-4x+1-8x^2+8=11`

`4x+13=11`

`4x=-2`

`x=-2/4=-1/2`

Vậy `S={-1/2}`

`b)`

`( 5x-1) ^2 - ( 5x-4) ( 5x+4) =7 `

`25x^2-10x+1-25x^2+16=7`

`-10x+17=7`

`-10x=-10`

`x=1`

Vậy `S={1}`

`c)`

` ( 2x + 3) ^2 -4 ( x-3) ^2 +3 ( x+2) ( x-2) =6`

`4x^2+12x+9-4x^2+24x-36+3x^2-12=6`

`3x^2+36x-39=6`

`3x^2+36x=45`

`\frac{3x^2+36x}{3}=\frac{45}{3}`

`x^2+12x=15`

`x^2+12x+36=15+36`

`(x+6)^2=51`

`x+6=\sqrt{51}` hoặc `x+6=-\sqrt{51}`

`x=\sqrt{51}-6` hoặc `x=-\sqrt{51}-6`

Vậy `S={\sqrt{51}-6 ;-\sqrt{51}-6}`