$\textit{x² + 5x + 6=0}$
$\textit{$\longrightarrow$ x² + 2x + 3x + 6 = 0}$
$\textit{$\longrightarrow$ x(x+2) + 3(x+2) =0 }$
$\textit{$\longrightarrow$ (x+2)(x+3)=0}$
$\textit{$\longrightarrow$ x+2 = 0 họặc x+3=0 }$
$\textit{$\longrightarrow$ x=-2 x=-3 }$
$\textit{Vậy phương trình có 2 nghiệm là S={-2;-3}}$