a,
`(x-3)(x+3)-x(x-4)=4`
`<=>x^2-3^2-x^2+4x-4=0`
`<=>x^2-9-x^2+4x-4=0`
`<=>4x-13=0`
`<=>4x=13`
`<=>x=13/4`
Vậy `S={13/4}`
b,
`(2x-5)(2x+5)-4x(x+5)=-7`
`<=>(2x)^2-5^2-4x^2-20x+7=0`
`<=>4x^2-25-4x^2-20x+7=0`
`<=>-20x-18=0`
`<=>-20x=18`
`<=>x=-9/10`
Vậy `S={-9/10}`