$\#phnn$
`a)3.(4x+5)=39`
`<=>4x+5=13`
`<=>4x=8`
`<=>x=2.`
`\text{Vậy S= {2}}`
`b)(x+3)(2x-7)`
`=2x^2-7x+6x-21`
`=2x^2-x-21.`
`c)(y+4)^2`
`=y^2+2.y.4+4^2`
`=y^2+8y+16.`
`d)(y-2)^2-3y(y-5)`
`=y^2-2.y.2+2^2-3y^2+15y`
`=y^2-4y+4-3y^2+15y`
`=-2y^2+4+15y.`
`e)(2x+3)(2x-3)-6x=(2x+1)^2`
`<=>(2x)^2-3^2-6x-(2x+1)^2=0`
`<=>4x^2-9-6x-[(2x)^2+2.2x.1+1^2]=0`
`<=>4x^2-9-6x-4x^2-4x-1=0`
`<=>-10x-10=0`
`<=>-10x=10`
`<=>x=-1.`
`\text{Vậy S = {-1}}`