`(3x+1)(4x+15)(2x+3)(6x+5)+104x^2=0`
`<=>144x^4+924x^3+1836x^2+1155x+225=0`
`<=>3(48x^4+308x^8+612x^2+385x+75)=0`
`<=>3(48x^3+164x^2+120x+25)(x+3)=0`
`<=>(48x^3+1464x^2+120x+25)(x+3)=0`
`<=>(2x+1)(12x+5)(2x+5)(x+3)=0` `
`<=>x=-1/2` hoặc `x=-5/12` hoặc `x=-5/2` hoặc `x=-3`
Vậy `S={-1/2;-5/12;-5/2;-3}`