`a)` `x^2+3x=0`
`<=>x(x+3)=0`
`<=>`$\left[\begin{matrix} x=0\\ x+3=0\end{matrix}\right.$
`<=>`$\left[\begin{matrix} x=0\\ x=-3\end{matrix}\right.$
`text{Vậy}` `S={0;-3}`
`b)` `x^3-4x=0`
`<=>x(x^2-4)=0`
`<=>x(x^2-2^2)=0`
`<=>x(x-2)(x+2)=0`
`<=>`$\left[\begin{matrix} x=0\\ x-2=0\\ x+2=0\end{matrix}\right.$
`<=>`$\left[\begin{matrix} x=0\\ x=2\\ x=-2\end{matrix}\right.$
`text{Vậy}` `S={0;2;-2}`
`c)` `x^2+5x=6`
`<=>x^2+5x-6=0`
`<=>x^2+6x-x-6=0`
`<=>x(x+6)-(x+6)=0`
`<=>(x+6)(x-1)=0`
`<=>`$\left[\begin{matrix} x+6=0\\ x-1=0\end{matrix}\right.$
`<=>`$\left[\begin{matrix} x=-6\\ x=1\end{matrix}\right.$
`text{Vậy}` `S={-6;1}`
`d)` `x^2-2015x+2014=0`
`<=>x^2-2014x-x+2014=0`
`<=>x(x-2014)-(x-2014)=0`
`<=>(x-2014)(x-1)=0`
`<=>`$\left[\begin{matrix} x-2014=0\\ x-1=0\end{matrix}\right.$
`<=>`$\left[\begin{matrix} x=2014\\ x=1\end{matrix}\right.$
`text{Vậy}` `S={2014;1}`
