`a) (2x+3)^3-(2x-3)(4x^2+6x+9)=36x^2`
`<=> 8x^3+36x^2+54x+27-(8x^3-27)=36x^2`
`<=> 8x^3+36x^2+54x+27-8x^3+27-36x^2=0`
`<=> 54x+54=0`
`<=> 54x=-54`
`<=> x=-1`
Vậy `S={-1}`
`b) (4x+1)(16x^2-4x+1)+(3-4x)^3-(12x+13)(12x-13)=-127`
`<=> 64x^3+1+27-108x+144x^2-64x^3-(144x^2-169)+127=0`
`<=> 144x^2-108x+28-144x^2+169+127=0`
`<=> -108x+324=0`
`<=> 108x=324`
`<=> x=3`
Vậy `S={3}`