x^2 + 3x - 180 =0 tìm x

`x^2 + 3x - 180 = 0` 
`<=> x^2 + 15x - 12x - 180 = 0` 
`<=> x . ( x + 15 ) - 12 . ( x + 15 ) = 0` 
`<=> ( x + 15 ) . ( x - 12 ) = 0` 
`<=>` \(\left[ \begin{array}{l}x+15=0\\x-12=0\end{array} \right.\) 
`<=>` \(\left[ \begin{array}{l}x=-15\\x=12\end{array} \right.\) 
Vậy `S = { -15 ; 12 }`